segmentation fault - core dump

hi all, could someone give me a description of what this really means? i know what it means in general, but in php, what are the likely candidates? i thougth i had it narrowed down to a warning in fopen and a bad fp, but i've taken care of that, and its still core dumping...i'm using the cgi version, it didn't seem to have this problem when accessed via teh web & apache...there's too much code to really include, so anyone who has a second and some thoughts on how this most likely occurs, your thoughts would be much appreciated.... thanks, jack