A core file records memmory image of running process.when a php process is killed, apache creates core files under your account.These core files takes too much of space on server.There is no harm on deleting these files.You cabn get rid off these core files by modifiying the httpd start up file.

Vi /etc/init.d/httpd

Search for ulimit lines ex:

ulimit -n 1024

ulimit -n 4096

ulimit -n 8192

ulimit -n 16384

Please add ulimit -c 0 to the end.So its looks like

ulimit -n 16384

ulimit -c 0

Save the changes and restart apache service.