Write and call a function

displayName();

function displayName() {
      echo "Client A";
}

Function with arguments (parameters)

addNumbers(10, 10)

function addNumbers($value1, $value2) {
      echo ($value1 + $value2);
}

Return value from the function

$answer = subtractNumbers(10, 10);
echo $answer;

function subtractNumbers($value1, $value2) {
      $result = ($value1 - $value2);
      return $result;
}

Strict type

declare(strict_types=1); // enable strict type
$answer = multiplyNumbers(10, 10); // must pass the value that will match with the argument type when strict type is enabled
echo $answer;

// This method receives int type values and the return type is also int i.e. : int {
function multiplyNumbers(int $value1, int $value2) : int {
      $product = ($value1 * $value2);
      return $product;
}

declare(strict_types=1); // enable strict type

$evenNumbersList = getEvenNumbersOnly(10);
echo var_dump ($evenNumbersList);

function getEvenNumbersOnly(int $value) : array {
      $evenNumbers = [];
      for ($i=1; $i <= $value; $i++) {
            if ($i % 2 == 0) {
                  $evenNumbers[] = $i;
            }
      }   

      return $evenNumbers;
}

Optional argument

displayRange(1);

function displayRange (int $start, int $end = 100) {
      echo  "Range: "$start . " to " . $end;
}

Dynamic Function

$doubleIt = "doubleTheValue";
$doubleIt (5);

function doubleTheValue (int $value) {
      echo ($value * 2);
}

Anonymous/Closure Function

$doubleIt(5);

$doubleIt = function (int $value) {
      echo ($value * 2);
};

$input = 10;      
$doubleIt = function() use ($input) { // anonymous function can access variable defined outside with the use keyword. 
      echo ($input * 2);
};

$doubleIt();

Arguments as reference

$input = "Good";
getGreetings();
echo $input

function (string &$input) {
      $input = $input . "Morning";
}