Represent callable strings as strings (#1741)

Co-authored-by: Alex Bouma <alex@bouma.me>
Co-authored-by: Michi Hoffmann <cleptric@users.noreply.github.com>

src/Serializer/AbstractSerializer.php

@@ -280,7 +280,7 @@ protected function serializeValue($value)
      */
     protected function serializeCallable($callable): string
     {
-        if (\is_string($callable) && !\function_exists($callable)) {
+        if (\is_string($callable)) {
             return $callable;
         }
 
@@ -292,7 +292,7 @@ protected function serializeCallable($callable): string
             if (\is_array($callable)) {
                 $reflection = new \ReflectionMethod($callable[0], $callable[1]);
                 $class = $reflection->getDeclaringClass();
-            } elseif ($callable instanceof \Closure || (\is_string($callable) && \function_exists($callable))) {
+            } elseif ($callable instanceof \Closure) {
                 $reflection = new \ReflectionFunction($callable);
                 $class = null;
             } elseif (\is_object($callable) && method_exists($callable, '__invoke')) {

tests/Serializer/AbstractSerializerTest.php

@@ -539,6 +539,11 @@ public function serializableCallableProvider(): array
                 'callable' => $callableWithoutNamespaces,
                 'expected' => 'Lambda void {closure} [int|null param1_70ns]',
             ],
+            [
+                // This is (a example of) a PHP provided function that is technically callable but we want to ignore that because it causes more false positives than it helps
+                'callable' => 'header',
+                'expected' => 'header',
+            ],
             [
                 'callable' => __METHOD__,
                 'expected' => __METHOD__,