Besides the real backward-compatibility breaks previously discussed, there are also a number of “deprecated” features. Deprecated features emit an E_STRICT error, which is not part of the E_ALL error setting. To see those deprecated issues in PHP 4 code, you need to set error reporting to E_ALL | E_STRICT.
Because PHP 4 does not understand the E_STRICT constant, you might want to use the numerical version to make the scripts run with both PHP 4 and PHP 5. The numerical value for E_STRICT is 2048. To show all errors (E_ALL and E_STRICT), you need to use the value 4095 for either the error_reporting() function or as php.ini setting.
In PHP 4, the following code would automagically create ...