What are some common issues that arise when transitioning from PHP4 to PHP 5.4?

One common issue when transitioning from PHP4 to PHP 5.4 is the removal of the call-time pass by reference feature. This means that functions can no longer be called with references unless the function specifically declares the parameter as a reference. To solve this, update the function definition to include the '&' symbol before the parameter name to indicate that it is a reference.

// Before PHP 5.4
function foo(&$var) {
    $var++;
}

// After PHP 5.4
function foo($var) {
    $var++;
}