Zephir\FunctionCall::functionExists PHP Method

    public function functionExists($functionName, CompilationContext $context)
    {
        if (function_exists($functionName)) {
            return true;
        }
        if ($this->isBuiltInFunction($functionName)) {
            return true;
        }
        $internalName = array('f__' . $functionName);
        if (isset($context->classDefinition)) {
            $internalName[] = 'f_' . str_replace('\\', '_', strtolower($context->classDefinition->getNamespace())) . '_' . $functionName;
        }
        foreach ($internalName as $name) {
            if (isset($context->compiler->functionDefinitions[$name])) {
                return true;
            }
        }
        return false;
    }