diff --git a/src/Php/Optimizer/FuncCallOptimizer.php b/src/Php/Optimizer/FuncCallOptimizer.php
index ef0d95c3..1407da95 100644
--- a/src/Php/Optimizer/FuncCallOptimizer.php
+++ b/src/Php/Optimizer/FuncCallOptimizer.php
@@ -393,6 +393,17 @@ trait FuncCallOptimizer
                 }
                 continue;
             }
+            // When null is explicitly passed to an optional parameter, skip it
+            // so the C++ default value takes effect (PHP null = "use default").
+            if ($optional && $argCount > $i && isset($expr->args[$i])) {
+                $argVal = $expr->args[$i]->value;
+                if ($argVal instanceof Node\Expr\ConstFetch && strtolower($argVal->name->toString()) === 'null') {
+                    if (isset($defaults[$i])) {
+                        $args[] = $defaults[$i];
+                    }
+                    continue;
+                }
+            }
             $args[] = $this->resolveArg($expr, $i, $type);
         }
 
diff --git a/tests/aot/stdlib/null_optional_arg.phpt b/tests/aot/stdlib/null_optional_arg.phpt
new file mode 100644
index 00000000..902320d9
--- /dev/null
+++ b/tests/aot/stdlib/null_optional_arg.phpt
@@ -0,0 +1,51 @@
+--TEST--
+Passing null to optional parameters should use C++ default values
+--FILE--
+<?php
+
+// substr: null length should take rest of string, not return empty
+$s = 'hello world';
+var_dump(substr($s, 6, null));
+var_dump(substr($s, 6) === substr($s, 6, null));
+var_dump(substr($s, 0, null) === $s);
+var_dump(substr($s, null) === $s);
+
+// strpos: null offset should default to 0
+var_dump(strpos($s, 'o', null));
+var_dump(strpos($s, 'o', null) === strpos($s, 'o'));
+
+// stripos: null offset should default to 0
+var_dump(stripos($s, 'O', null));
+var_dump(stripos($s, 'O', null) === stripos($s, 'O'));
+
+// strrpos: null offset should default to 0 (search from end)
+var_dump(strrpos('hello hello', 'o', null));
+var_dump(strrpos('hello hello', 'o', null) === strrpos('hello hello', 'o'));
+
+// strstr: null before_needle should default to false
+var_dump(strstr($s, 'o', null));
+var_dump(strstr($s, 'o', null) === strstr($s, 'o'));
+
+// str_repeat: ensure non-null still works
+var_dump(str_repeat('ab', 3));
+
+// explode with null limit should default to PHP_INT_MAX (no limit)
+$arr = explode(' ', 'a b c d', null);
+var_dump(count($arr));
+
+?>
+--EXPECT--
+string(5) "world"
+bool(true)
+bool(true)
+bool(true)
+int(4)
+bool(true)
+int(4)
+bool(true)
+int(10)
+bool(true)
+string(7) "o world"
+bool(true)
+string(6) "ababab"
+int(4)