Possible hacker using php script to send e-mails?

Hi there, I am running php 4.x on a suse 9.x machine. There is a php script which resides on a webapp that is responsible for sending e-mail to myself in case of errors like db-errors or similar. Called error.php This script does include phpmailer and uses it to send the e-mails to me. Now I am receiving on the e-mail specified as TO: e-mails with different subject than specified and different text?! All english text with wired sentences, must be a bot or so. How is this possible? The subject line is fixed and right after that commend send is executed. So no idea how they do it and how I can prevent it. It looks like this: $mail->Subject = 'Fehlerbericht'; $mail->Send(); How is it possible that they change this subject line? I checked the server log and each time an e-mail has been sent to me of that kind there is a logentry in apache log that says that this script has been executed. So the e-mails definatelly come from that script?! Can anybody help? Regards, Merlin