What are the key differences between PHP 5.x and PHP 7 that developers should be aware of?

One key difference between PHP 5.x and PHP 7 is the performance improvement in PHP 7 due to the introduction of the Zend Engine 3. This results in faster execution times and reduced memory usage. Additionally, PHP 7 introduces scalar type declarations, return type declarations, and the spaceship operator (<=>) for easier and more robust coding.

// PHP 5.x code
function sum($a, $b){
    return $a + $b;
}

// PHP 7 code with scalar type declarations and return type declarations
function sum(int $a, int $b): int {
    return $a + $b;
}