PHP and permissions

Hi, I have a question about how PHP runs. I have a file on my FreeBSD server that is owned by another user but is the same group as myself. The file has read/write permissions for the user only. Therefore, when I log onto the server I cannot read or write to the file. In fact, I can not even FTP it. The funny thing is, I wrote a PHP script to read the file (it's actually a CSV file) into an array using the function fgetcsv and display the values within the browser. This worked. So my question is, what user is PHP run under that allows me to access another user's file (but the same group)? Thanks, Don