The PHP interpreter is commonly used in cron jobs to execute php code. All our servers run a dual PHP setup so the common command of

php /home/me/public_html/cron.php

for example will execute under PHP5. Here are the paths to both interpreters

PHP4: /usr/local/php4/bin/php
PHP5: /usr/local/bin/php

You will replace php in your cron with either of those locations to run that specific PHP version.