If you have PHP installed as a command line tool (try issuing php to the terminal and see if it works), your shebang (#!) line needs to look like this:
#!/usr/bin/php
Put that at the top of your script, make it executable (chmod +x myscript.php), and make a Cron job to execute that script (same way you’d execute a bash script).
You can also use php myscript.php.