500 Internal Server Error for php file not for html [duplicate]

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!

I was having this problem because I was trying to connect to MySQL but I didn't have the required package. I figured it out because of @Amadan's comment to check the error log. In my case, I was having the error: Call to undefined function mysql_connect()

If your PHP file has any code to connect with a My-SQL db then you might need to install php5-mysql first. I was getting this error because I hadn't installed it. All my file permissions were good. In Ubuntu, you can install it by the following command:

sudo apt-get install php5-mysql