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

It could come from numerous sources as explained in your other topic...

FTP is likely (so check FTP logs). Or some code in your website allows a visitor to upload file, where your code does not validate that the uploaded file is an image for example so someone could upload a .php file.