PHP - recognize when the function was called

[undusioli]

I'm thinking about how to find from where any function was called. The problem is that I need to find where the PHP is calling \[code\]mail()\[/code\] function. One way will be to use \[code\]register_tick_function()\[/code\], but I'll need to open each file and check what is on each line. The project is huge, it will take really long to parse each file in PHP. Any other way? Or option how to override the \[code\]mail()\[/code\] function?