When adding a PHP script as a cron you need to ensure that you include the full PHP directory, simply running the following will not allow the script to run:
30 6 * * * php -q public_html/clients/scripts/mycron.php
Using the following will allow the PHP script to be used within a cron process:
30 6 * * * /usr/local/bin/php -q /home/username/public_html/clients/admin/mycron.php
Note how we need to utilise the full directory structure to ensure both the PHP process is called correctly and has the full directory of your PHP script.