how to resolve 500 internal server error for my php application?

Question

I am getting 500 internal server error as mentioned below but i have the mentioned file located at /var/www/html/mywebsite/new_catalogue_request.php

Request URL: https://www.mywebsite.com/custom_vm/new_catalogue_request.php
Request Method: POST
Status Code: 500 Internal Server Error
Remote Address: xxx.xxx.xxx.xxx:443
Referrer Policy: strict-origin-when-cross-origin

Below are the tools details:

1. PHP Version- 5.3
2. Joomla Version- 2.5
3. Apache- 2.4

Efforts:

1. I Checked the apache and other possible error logs but dint get much info.
2. Tried executing the command "chmod -R 755 /mywebsite" but dint help much.

Please help in resolving this issue.