With PHP 3 the error reporting level was set as a simple numeric value formed by summing up the numbers related to different error levels. Usual values were 15 for reporting all errors and warnings or 7 for reporting everything but simple notice messages reporting bad style and things like that.
PHP 4 has a larger set of error and warning levels and comes with a configuration parser that now allows for symbolic constants to be used for setting the intended behavior.
Error reporting level should now be configured by explicitly taking away the warning levels you do not want to generate error messages by x-oring them from the symbolic constant E_ALL. Sounds complicated? Well, lets say you want the error reporting system to report all but the simple style warnings that are categorized by the symbolic constant E_NOTICE. Then you'll put the following into your php.ini: error_reporting = E_ALL & ~ ( E_NOTICE ). If you want to suppress warnings too you add up the appropriate constant within the braces using the binary or operator '|': error_reporting= E_ALL & ~ ( E_NOTICE | E_WARNING ).
When upgrading code or servers from PHP 3 to PHP 4 you should check these settings and calls to error_reporting() or you might disable reporting the new error types, especially E_COMPILE_ERROR. This may lead to empty documents without any feedback of what happened or where to look for the problem.
Using the old values 7 and 15 for setting up error reporting is a very bad idea as this suppresses some of the newly added error classes including parse errors. This may lead to very strange behavior as scripts might no longer work without error messages showing up anywhere.
This has lead to a lot of unreproducible bug reports in the past where people reported script engine problems they were not capable to track down while the true case was usually some missing '}' in a required file that the parser was not able to report due to a misconfigured error reporting system.
So checking your error reporting setup should be the first thing to do whenever your scripts silently die. The Zend engine can be considered mature enough nowadays to not cause this kind of strange behavior.
A lot of existing PHP 3 code uses language constructs that should be considered as very bad style as this code, while doing the intended thing now, could easily be broken by changes in other places. PHP 4 will output a lot of notice messages in such situations where PHP 3 didn't. The easy fix is to just turn off E_NOTICE messages, but it is usually a good idea to fix the code instead.
The most common case that will now produce notice messages is the use of unquoted string constants as array indices. Both PHP 3 and 4 will fall back to interpret these as strings if no keyword or constant is known by that name, but whenever a constant by that name had been defined anywhere else in the code it might break your script. This can even become a security risk if some intruder manages to redefine string constants in a way that makes your script give him access rights he wasn't intended to have. So PHP 4 will now warn you whenever you use unquoted string constants as for example in $_SERVER[REQUEST_METHOD]. Changing it to $_SERVER['REQUEST_METHOD'] will make the parser happy and greatly improve the style and security of your code.
Another thing PHP 4 will now tell you about is the use of uninitialized variables or array elements.