Fatal Error: Call to undefined function

[ChopBam]

Hello all.
Whenever anybody tries to view a thread or a profile, this fatal error comes up:

Fatal error: Call to undefined function: str_ireplace() in /home/warrior/public_html/forums/includes/class_postbit.php(296) : eval()'d code on line 12

I can't figure it out for the life of me, and it's a pretty major error, since nobody can view threads or anything. I'm not up to speed on how PHP works, so go easy on the terminology. ;)
Thanks in advance to anybody who can help.

Edit: sorry if this is in the wrong section. I made it hasty.

[Paul M]

You have a modification installed that is causing this - you need to disable them until you find which one it is.

[ChopBam]

Okay I found the problem. It was indeed one of the hacks. Thank you very much.

[Marco van Herwaarden]

str_ireplace() is a PHP 5 function. The modification you have installed probably doesnot take into account that it can also be used on PHP4 installations (should be tested in the install routine).