Links Files In Linux (file.txt For File.php)

Today I found some cstomer on the servers make a link for named it file.txt and link it to other customer php file.

so that customer have the ability to show the other custoer file content when visiting the url because it is a text wile originally it is a php file.

the php file was a config file, so now he know the database password , and because he is in the same server he can use that databse.

the question , how to avoide this prolem in the future?

notes , the SuExec is rnning and the open_basedir protection is enabled, but the problem still exists.