How to make people aware of why something might not work.

verison_check.php

<?php
# Making use of: http://php.net/manual/en/function.phpversion.php
#
# The idea is you'd stick this in your core class at the top to prevent execution
# on out-dated runtime environments.
#
# You could use 50300 if you only care about 5.3, but it's EOL too, so...
if (!defined('PHP_VERSION_ID')) || PHP_VERSION_ID < 50400) {
  # Inform users they need to upgrade their PHP version (or have their host do so)
  # It'd be great if there was a common message or link to send users.
  # Don't let anything else execute.
  return;
}

# Your normal code starts here.