Debuging PHP script

Hi, I installed v-webmail on the PHP engine to enable web mail client access. Every installation step was going well. But, I found a problem when the application was using for awhile. My webmail can display the login page, but when I entered login/password and press submit. I got an error display that 'can't find server' and there was no PHP error displaying. But, there was a 'Exit segmentation fault (11)" error in httpd log file, everytime I press submit button in login page. I tried to put a debug line to determine the statement that cause the error, but very difficult to do. Does PHP has any directive or command to debug a code or print all success/unsuccess line execution. I can't modify php.ini or httpd.conf, because that server is not mine (I use a hosting service from ISP). Thanks in advance, Skon L.