500 Internal Server Error is shown if your php code has fatal errors but error displaying is switched off. You may try this to see the error itself instead of 500 error page:
In your php file:
ini_set(‘display_errors’, 1);
In .htaccess file:
php_flag display_errors 1
A PHP file must have permissions set to 644. Any folder containing PHP files and PHP access (to upload files, for example) must have permissions set to 755. PHP will run a 500 error when dealing with any file or folder that has permissions set to 777!