When troubleshooting a 500 Internal Server Error with PHP files on a Linux hosting environment you will want to review the .htaccess file in the existing directory as well as any directories above. First you can test the site by disabling the .htaccess file, this can be done by renaming the file (i.e. htaccessbak). This will make the file not readable as a configuration file by the server, it is then recommended that you review your site and complete a hard refresh or clear the browsers cache and cookies and reload the page.
If the error in question goes away then the resulting issue is that a configuration from within your .htaccess file is causing this issue. You will need to review the lines of code in your .htaccess file to determine where the issue lies, leaving the file disabled may disable other features in your website that are necessary. Unfortunately this task may be tedious depending on how much content is in the file, you may wish to start with the most recent changes to your site to determine if any of this information was to the .htaccess file. You can use a semicolon in front of the line of code to block out the information from being read and save the file to complete another hard fresh of your site.