The command could be something like:
/usr/bin/php -f /home/(user)/public_html/phptorun.php
You need to confirm which directory the PHP executable is in, and adjust the "/usr/bin/php" portion accordingly.
Alternatively, you can make sure the php file has execute permission, and use the "shebang" line at the top of the file so that the system knows how to run it:
#!/usr/bin/php
<?php
// . . .
?>
Then you could just specify the PHP file's path in the cron tool instead (without the path to the PHP executable).