How can PHP functions be optimized for better performance when generating code?

// Example of optimizing PHP functions for better performance when generating code

// Bad practice: using unnecessary function calls
function calculateSum($a, $b) {
    return $a + $b;
}

// Good practice: directly performing the operation
$sum = $a + $b;

// Bad practice: using global variables
$globalVar = 10;

function calculateProduct($a) {
    global $globalVar;
    return $a * $globalVar;
}

// Good practice: passing variables as arguments
function calculateProduct($a, $globalVar) {
    return $a * $globalVar;
}

// Bad practice: using custom functions instead of built-in PHP functions
function customFunction($array) {
    $sum = 0;
    foreach ($array as $value) {
        $sum += $value;
    }
    return $sum;
}

// Good practice: using built-in PHP functions
$sum = array_sum($array);

// Bad practice: not caching expensive function calls
function calculateExpensiveOperation($a) {
    // Expensive operation
    return $result;
}

// Good practice: caching the result
$result = calculateExpensiveOperation($a);