someone is trying to change my ci index file how to make it secure

[13-Aug-2017 11:27:58 PRC] PHP Warning:  preg_replace() [<a href='function.preg-replace'>function.preg-replace</a>]: Compilation failed: unmatched parentheses at offset 17 in /home/nmcth/public_html/index.php(12) : runtime-created function on line 1

[13-Aug-2017 11:27:58 PRC] PHP Warning:  strlen() expects parameter 1 to be string, array given in /home/nmcth/public_html/index.php(12) : runtime-created function on line 1

i found this in my error_log file. how can i fix it and know from which root it has been done