diff --git a/lib/Compiler.php b/lib/Compiler.php
index e2da6e10..e5cfe665 100755
--- a/lib/Compiler.php
+++ b/lib/Compiler.php
@@ -150,11 +150,15 @@ protected function compileTypeConstrainedVariable(Block $block, Type $type): int
     protected function compileParam(Op\Expr\Param $param, Block $block, int $paramIdx): OpCode {
         assert(false === $param->byRef);
         assert(false === $param->variadic);
-        assert(null === $param->defaultBlock);
+        $defaultConst = null;
+        if (null !== $param->defaultVar) {
+            $defaultConst = $this->compileOperand($param->defaultVar, $block, true);
+        }
         return new OpCode(
             OpCode::TYPE_ARG_RECV,
             $this->compileOperand($param->result, $block, false),
-            $paramIdx
+            $paramIdx,
+            $defaultConst
         );
     }
 
@@ -655,8 +659,24 @@ protected function compileOperand(Operand $operand, Block $block, bool $isRead):
             return null;
         } elseif ($operand instanceof Operand\Literal) {
             assert($isRead === true);
-            $return = new Variable(Variable::mapFromType($operand->type));
-            switch (Variable::mapFromType($operand->type)) {
+            $mappedType = null !== $operand->type
+                ? Variable::mapFromType($operand->type)
+                : Variable::TYPE_UNDEFINED;
+            if ($mappedType === Variable::TYPE_UNDEFINED) {
+                if (is_int($operand->value)) {
+                    $mappedType = Variable::TYPE_INTEGER;
+                } elseif (is_float($operand->value)) {
+                    $mappedType = Variable::TYPE_FLOAT;
+                } elseif (is_string($operand->value)) {
+                    $mappedType = Variable::TYPE_STRING;
+                } elseif (is_bool($operand->value)) {
+                    $mappedType = Variable::TYPE_BOOLEAN;
+                } elseif (null === $operand->value) {
+                    $mappedType = Variable::TYPE_NULL;
+                }
+            }
+            $return = new Variable($mappedType);
+            switch ($mappedType) {
                 case Variable::TYPE_STRING:
                     $return->string($operand->value);
                     break;
@@ -669,8 +689,10 @@ protected function compileOperand(Operand $operand, Block $block, bool $isRead):
                 case Variable::TYPE_BOOLEAN:
                     $return->bool($operand->value);
                     break;
+                case Variable::TYPE_NULL:
+                    break;
                 default:
-                    throw new \LogicException("Unknown Literal Operand Type: " . $operand->type);
+                    throw new \LogicException('Unknown Literal Operand Type: ' . ($operand->type ?? 'untyped'));
             }
             return $block->registerConstant($operand, $return);
         } elseif ($operand instanceof Operand\Temporary) {
diff --git a/lib/Frame.php b/lib/Frame.php
index 13c7b0bd..215513ee 100755
--- a/lib/Frame.php
+++ b/lib/Frame.php
@@ -26,6 +26,9 @@ class Frame {
     public ?Variable $returnVar = null;
     public ?Handler $handler = null;
 
+    /** When true, finishing this frame resumes the caller instead of ending execution. */
+    public bool $ephemeral = false;
+
     public function __construct(?Handler $handler, ?Block $block, ?Frame $parent, Variable ...$scope) {
         $this->handler = $handler;
         $this->block = $block;
diff --git a/lib/JIT.php b/lib/JIT.php
index 878ef839..c2b2ab78 100644
--- a/lib/JIT.php
+++ b/lib/JIT.php
@@ -145,7 +145,8 @@ private function compileBlock(Block $block, ?string $funcName = null): PHPLLVM\V
             if ($isVarArgs) {
                 $this->context->functionProxies[$lcname] = new JIT\Call\Vararg($func, $funcName, count($args));
             } else {
-                $this->context->functionProxies[$lcname] = new JIT\Call\Native($func, $funcName, $args);
+                $defaultArgs = $this->collectParamDefaults($block);
+                $this->context->functionProxies[$lcname] = new JIT\Call\Native($func, $funcName, $args, $defaultArgs);
             }
         }
 
@@ -727,4 +728,42 @@ private function jitTypeFromLlvmValue(PHPLLVM\Value $value): int
         }
     }
 
+    /**
+     * @return array<int, Variable>
+     */
+    private function collectParamDefaults(Block $block): array {
+        $defaults = [];
+        foreach ($block->opCodes as $op) {
+            if ($op->type !== OpCode::TYPE_ARG_RECV || null === $op->arg3) {
+                continue;
+            }
+            if (!isset($block->constants[$op->arg3])) {
+                continue;
+            }
+            $defaults[$op->arg2] = $this->jitVariableFromVmConstant($block->constants[$op->arg3]);
+        }
+        return $defaults;
+    }
+
+    private function jitVariableFromVmConstant(VM\Variable $vm): Variable {
+        switch ($vm->type) {
+            case VM\Variable::TYPE_INTEGER:
+                return Variable::fromConstantInt($this->context, $vm->toInt());
+            case VM\Variable::TYPE_STRING:
+                $lit = new Operand\Literal($vm->toString());
+                $lit->type = Type::string();
+                return Variable::fromLiteral($this->context, $lit);
+            case VM\Variable::TYPE_FLOAT:
+                $lit = new Operand\Literal($vm->toFloat());
+                $lit->type = Type::float();
+                return Variable::fromLiteral($this->context, $lit);
+            case VM\Variable::TYPE_BOOLEAN:
+                $lit = new Operand\Literal($vm->toBool());
+                $lit->type = Type::bool();
+                return Variable::fromLiteral($this->context, $lit);
+            default:
+                throw new \LogicException('Unsupported default parameter type for JIT');
+        }
+    }
+
 }
diff --git a/lib/JIT.pre b/lib/JIT.pre
index aff76d38..00f43c6b 100755
--- a/lib/JIT.pre
+++ b/lib/JIT.pre
@@ -142,7 +142,8 @@ class JIT {
             if ($isVarArgs) {
                 $this->context->functionProxies[$lcname] = new JIT\Call\Vararg($func, $funcName, count($args));
             } else {
-                $this->context->functionProxies[$lcname] = new JIT\Call\Native($func, $funcName, $args);
+                $defaultArgs = $this->collectParamDefaults($block);
+                $this->context->functionProxies[$lcname] = new JIT\Call\Native($func, $funcName, $args, $defaultArgs);
             }
         }
 
@@ -609,4 +610,42 @@ class JIT {
         }
     }
 
+    /**
+     * @return array<int, Variable>
+     */
+    private function collectParamDefaults(Block $block): array {
+        $defaults = [];
+        foreach ($block->opCodes as $op) {
+            if ($op->type !== OpCode::TYPE_ARG_RECV || null === $op->arg3) {
+                continue;
+            }
+            if (!isset($block->constants[$op->arg3])) {
+                continue;
+            }
+            $defaults[$op->arg2] = $this->jitVariableFromVmConstant($block->constants[$op->arg3]);
+        }
+        return $defaults;
+    }
+
+    private function jitVariableFromVmConstant(VM\Variable $vm): Variable {
+        switch ($vm->type) {
+            case VM\Variable::TYPE_INTEGER:
+                return Variable::fromConstantInt($this->context, $vm->toInt());
+            case VM\Variable::TYPE_STRING:
+                $lit = new Operand\Literal($vm->toString());
+                $lit->type = Type::string();
+                return Variable::fromLiteral($this->context, $lit);
+            case VM\Variable::TYPE_FLOAT:
+                $lit = new Operand\Literal($vm->toFloat());
+                $lit->type = Type::float();
+                return Variable::fromLiteral($this->context, $lit);
+            case VM\Variable::TYPE_BOOLEAN:
+                $lit = new Operand\Literal($vm->toBool());
+                $lit->type = Type::bool();
+                return Variable::fromLiteral($this->context, $lit);
+            default:
+                throw new \LogicException('Unsupported default parameter type for JIT');
+        }
+    }
+
 }
diff --git a/lib/JIT/Call/Native.php b/lib/JIT/Call/Native.php
index ddc62c62..834d32cc 100644
--- a/lib/JIT/Call/Native.php
+++ b/lib/JIT/Call/Native.php
@@ -24,15 +24,27 @@ class Native implements Call {
     public string $name;
     public array $argTypes;
 
-    public function __construct(Value $function, string $name, array $argTypes) {
+    /** @var array<int, Variable> compile-time defaults for optional parameters */
+    public array $defaultArgs = [];
+
+    public function __construct(Value $function, string $name, array $argTypes, array $defaultArgs = []) {
         $this->function = $function;
         $this->name = $name;
         $this->argTypes = $argTypes;
+        $this->defaultArgs = $defaultArgs;
     }
 
     public function call(Context $context, Variable ... $args): Value {
         $argValues = [];
-        foreach ($args as $index => $arg) {
+        $total = count($this->argTypes);
+        for ($index = 0; $index < $total; $index++) {
+            if (isset($args[$index])) {
+                $arg = $args[$index];
+            } elseif (isset($this->defaultArgs[$index])) {
+                $arg = $this->defaultArgs[$index];
+            } else {
+                throw new \LogicException("Missing required argument {$index} for {$this->name}()");
+            }
             $argValues[] = $this->compileArg($context, $arg, $index);
         }
         return $context->builder->call(
diff --git a/lib/JIT/Call/Native.pre b/lib/JIT/Call/Native.pre
index 1e64102d..a051c9e3 100755
--- a/lib/JIT/Call/Native.pre
+++ b/lib/JIT/Call/Native.pre
@@ -21,15 +21,27 @@ class Native implements Call {
     public string $name;
     public array $argTypes;
 
-    public function __construct(Value $function, string $name, array $argTypes) {
+    /** @var array<int, Variable> compile-time defaults for optional parameters */
+    public array $defaultArgs = [];
+
+    public function __construct(Value $function, string $name, array $argTypes, array $defaultArgs = []) {
         $this->function = $function;
         $this->name = $name;
         $this->argTypes = $argTypes;
+        $this->defaultArgs = $defaultArgs;
     }
 
     public function call(Context $context, Variable ... $args): Value {
         $argValues = [];
-        foreach ($args as $index => $arg) {
+        $total = count($this->argTypes);
+        for ($index = 0; $index < $total; $index++) {
+            if (isset($args[$index])) {
+                $arg = $args[$index];
+            } elseif (isset($this->defaultArgs[$index])) {
+                $arg = $this->defaultArgs[$index];
+            } else {
+                throw new \LogicException("Missing required argument {$index} for {$this->name}()");
+            }
             $argValues[] = $this->compileArg($context, $arg, $index);
         }
         return $context->builder->call(
diff --git a/lib/VM.php b/lib/VM.php
index d358a3a3..76d65c35 100755
--- a/lib/VM.php
+++ b/lib/VM.php
@@ -240,8 +240,15 @@ public function run(Block $block): int {
                 case OpCode::TYPE_ARG_RECV:
                     // Todo: do type checks and transformations
                     $arg1 = $frame->scope[$op->arg1];
-                    $arg1->copyFrom($frame->calledArgs[$op->arg2]);
-                    break;
+                    if (array_key_exists($op->arg2, $frame->calledArgs)) {
+                        $arg1->copyFrom($frame->calledArgs[$op->arg2]);
+                        break;
+                    }
+                    if (null !== $op->arg3 && isset($frame->block->constants[$op->arg3])) {
+                        $arg1->copyFrom($frame->block->constants[$op->arg3]);
+                        break;
+                    }
+                    throw new \LogicException('Missing required argument ' . $op->arg2);
                 case OpCode::TYPE_DECLARE_CLASS:
                     $name = $frame->scope[$op->arg1]->toString();
                     $lcname = strtolower($name);
@@ -331,6 +338,9 @@ public function run(Block $block): int {
                     throw new \LogicException("VM OpCode Not Implemented: " . $op->getType());
             }
         }
+        if ($frame->ephemeral) {
+            goto nextframe;
+        }
         return self::SUCCESS;
     }
 
diff --git a/patches/php-cfg-mixed-reserved.patch b/patches/php-cfg-mixed-reserved.patch
new file mode 100644
index 00000000..f3b9c74f
--- /dev/null
+++ b/patches/php-cfg-mixed-reserved.patch
@@ -0,0 +1,32 @@
+--- a/lib/PHPCfg/Op/Type/Mixed.php
++++ b/lib/PHPCfg/Op/Type/Mixed_.php
+@@ -13,5 +13,5 @@ namespace PHPCfg\Op\Type;
+ 
+ use PHPCfg\Op\Type;
+ 
+-class Mixed extends Type
++class Mixed_ extends Type
+ {
+ }
+--- a/lib/PHPCfg/Parser.php
++++ b/lib/PHPCfg/Parser.php
+@@ -173,7 +173,7 @@ class Parser
+             return new Op\Type\Literal($type);
+         }
+ 
+-        return new Op\Type\Mixed();
++        return new Op\Type\Mixed_();
+     }
+ 
+     private function processType(?Node $type): Op\Type
+--- a/lib/PHPCfg/Printer.php
++++ b/lib/PHPCfg/Printer.php
+@@ -246,7 +246,7 @@ class Printer
+         if ($type instanceof Op\Type\Literal) {
+             return $type->name;
+         }
+-        if ($type instanceof Op\Type\Mixed) {
++        if ($type instanceof Op\Type\Mixed_) {
+             return 'mixed';
+         }
+         if ($type instanceof Op\Type\Nullable) {
diff --git a/patches/php-types-mixed-reserved.patch b/patches/php-types-mixed-reserved.patch
new file mode 100644
index 00000000..f8a6e5c0
--- /dev/null
+++ b/patches/php-types-mixed-reserved.patch
@@ -0,0 +1,57 @@
+--- a/lib/PHPTypes/Type.php
++++ b/lib/PHPTypes/Type.php
+@@ -293,6 +293,9 @@ class Type
+         if ($decl instanceof CfgType\Literal) {
+             return self::fromDecl($decl->name);
+         }
++        if ($decl instanceof CfgType\Mixed_) {
++            return self::mixed();
++        }
+ 
+         throw new \LogicException('Unsupported declaration type: '.get_class($decl));
+     }
+--- a/lib/PHPTypes/TypeReconstructor.php
++++ b/lib/PHPTypes/TypeReconstructor.php
+@@ -393,9 +393,14 @@ class TypeReconstructor
+     protected function resolveOp_Expr_Param(Operand $var, Op\Expr\Param $op, SplObjectStorage $resolved)
+     {
+         $type = $this->resolveOpType($op->declaredType);
+-        if ($op->defaultVar) {
+-            if ($op->defaultBlock->children[0]->getType() === 'Expr_ConstFetch' && strtolower($op->defaultBlock->children[0]->name->value) === 'null') {
++        if ($op->defaultVar && $op->defaultBlock && isset($op->defaultBlock->children[0])) {
++            $defaultOp = $op->defaultBlock->children[0];
++            if (
++                method_exists($defaultOp, 'getType')
++                && $defaultOp->getType() === 'Expr_ConstFetch'
++                && strtolower($defaultOp->name->value) === 'null'
++            ) {
+                 $type = (new Type(Type::TYPE_UNION, [$type, Type::null()]))->simplify();
+             }
+         }
+@@ -541,7 +546,7 @@ class TypeReconstructor
+         foreach ($this->state->classes as $class) {
+             foreach ($class->stmts->children as $stmt) {
+                 if ($stmt instanceof Op\Stmt\Property) {
+-                    if ($stmt->declaredType instanceof Op\Type\Mixed) {
++                    if ($stmt->declaredType instanceof Op\Type\Mixed_) {
+                         $stmt->type = Type::extractTypeFromComment('var', $stmt->getAttribute('doccomment'));
+                     } else {
+                         $stmt->type = $this->resolveOpType($stmt->declaredType);
+@@ -677,7 +682,7 @@ class TypeReconstructor
+             foreach ($class->stmts->children as $property) {
+                 if ($property instanceof Op\Stmt\Property) {
+                     $property->type = $this->resolveOpType($property->declaredType);
+-                    if ($property->declaredType instanceof Op\Type\Mixed && $property->type) {
++                    if ($property->declaredType instanceof Op\Type\Mixed_ && $property->type) {
+                         $property->declaredType = new Op\Type\Literal($property->type->toDecl());
+                     }
+                 }
+@@ -694,7 +699,7 @@ class TypeReconstructor
+ 
+     private function resolveOpType(Op\Type $type): Type
+     {
+-        if ($type instanceof Op\Type\Mixed) {
++        if ($type instanceof Op\Type\Mixed_) {
+             return Type::mixed();
+         }
+         if ($type instanceof Op\Type\Nullable) {
diff --git a/script/apply-patches.sh b/script/apply-patches.sh
index 41c5add8..47a985f0 100755
--- a/script/apply-patches.sh
+++ b/script/apply-patches.sh
@@ -36,10 +36,12 @@ if [[ -d "$ROOT/vendor/ircmaxell/php-types" ]]; then
   apply_patch "$PATCH_DIR/php-types-binaryop-spaceship.patch"
   apply_patch "$PATCH_DIR/php-types-str-bool-fns.patch"
   apply_patch "$PATCH_DIR/php-types-dollars-brace.patch"
+  apply_patch "$PATCH_DIR/php-types-mixed-reserved.patch"
 fi
 
 if [[ -d "$ROOT/vendor/ircmaxell/php-cfg" ]]; then
   apply_patch "$PATCH_DIR/php-cfg-dollars-brace.patch"
+  apply_patch "$PATCH_DIR/php-cfg-mixed-reserved.patch"
 fi
 
 if [[ -d "$ROOT/vendor/pre/plugin" ]]; then
diff --git a/test/compliance/cases/language/default_params.phpt b/test/compliance/cases/language/default_params.phpt
new file mode 100644
index 00000000..a9cd06bc
--- /dev/null
+++ b/test/compliance/cases/language/default_params.phpt
@@ -0,0 +1,14 @@
+--TEST--
+Default parameter values and optional arguments
+--FILE--
+<?php
+function f($a = 1, $b = 2) {
+    return $a + $b;
+}
+echo f(), "\n";
+echo f(10), "\n";
+echo f(10, 20), "\n";
+--EXPECT--
+3
+12
+30
diff --git a/test/compliance/cases/language/default_params_jit.phpt b/test/compliance/cases/language/default_params_jit.phpt
new file mode 100644
index 00000000..383eda65
--- /dev/null
+++ b/test/compliance/cases/language/default_params_jit.phpt
@@ -0,0 +1,14 @@
+--TEST--
+Default parameter values and optional arguments (JIT)
+--FILE--
+<?php
+function f($a = 1, $b = 2) {
+    return $a + $b;
+}
+echo f(), "\n";
+echo f(10), "\n";
+echo f(10, 20), "\n";
+--EXPECT--
+3
+12
+30