使用php为特定用户分配读写文件权限

I want to change file permission for a particular user for a specific file . All these things needs to be done by php. I am using chmod(file_path, 0644). But its not working.

Please suggest.

missing "$" ?

chmod($file_path, 0644)

or no permission because not owner of $file_path (apache-php runs as user wwwrun)