What are common issues when upgrading from PHP 5.3 to PHP 5.4, especially with regard to passing variables by reference?

When upgrading from PHP 5.3 to PHP 5.4, a common issue with passing variables by reference arises due to stricter standards in PHP 5.4. To solve this issue, you need to ensure that variables passed by reference are explicitly declared as such using the `&` symbol in both the function definition and function call.

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

$bar = 5;
foo($bar);

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

$bar = 5;
foo($bar);