The chmod command takes the following parameters: 

chmod($mode, $filename);

You can pass more than one file name. 
The $mode value is a numeric value based on UNIX file permissions
The $mode value is usually specified in octal notation. 

To mark a file as read-only, you can use the following command: 
chmod(0444, $filename);

UNIX File Permissions is listed in the following table.

Value     Meaning
0400     Owner of file has read permission.
0200     Owner has write permission.
0100     Owner has execute permission.
0040     All users in same group have read permission.
0020     Group users have write permission.
0010     Group users have execute permission.
0004     All users have read permission.
0002     All users have write permission.
0001     All users have execute permission.
The leading zero means these values are octal.