It only work if your php support CLI.

/usr/bin/php -q /home/xxxxx/public_html/convert.php 59

On some servers CLI version of php will be in different folders, so you need to put correct path to PHP CLI in admin area.

Here is result on cpanel server


Code:
# /usr/local/bin/php -v
PHP 5.2.5 (cli) (built: Nov 14 2007 18:35:55)
Copyright (c) 1997-2007 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2007 Zend Technologies
    with Zend Extension Manager v1.2.0, Copyright (c) 2003-2007, by Zend Technologies
    with Zend Optimizer v3.3.0, Copyright (c) 1998-2007, by Zend Technologies

# /usr/bin/php -v
PHP 5.2.5 (cgi-fcgi) (built: Nov 14 2007 18:40:00)
Copyright (c) 1997-2007 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2007 Zend Technologies
    with Zend Extension Manager v1.2.0, Copyright (c) 2003-2007, by Zend Technologies
    with Zend Optimizer v3.3.0, Copyright (c) 1998-2007, by Zend Technologies
#
In the case, you need to use /usr/local/bin/php which is CLI