A function gets information from outside via arguments. You can define any number of arguments—including 0 (none). These arguments need at least a name so they can be used inside the function; there cannot be two arguments with the same name. When invoking the function, you need to send the arguments in the same order as declared.

A function may contain optional arguments, that is, you are not forced to provide a value for those arguments. When declaring the function, you need to provide a default value for those arguments. So, in case the user does not provide a value, the function will use the default one.

function addNumbers($a, $b, $printResult = false) {
    $sum = $a + $b;
    if ($printResult) {
        echo 'The result is ' . $sum;
    }
    return $sum;
}

$sum1 = addNumbers(1, 2);
$sum1 = addNumbers(3, 4, false);
$sum1 = addNumbers(5, 6, true); // it will print the result

This new function in the last example takes two mandatory arguments and an optional one. The default value of the optional argument is false, and it is then used normally inside the function. The function will print the result of the sum if the user provides true as the third argument, which happens only the third time that the function is invoked. For the first two, $printResult is set to false.

The arguments that the function receives are just copies of the values that the user provided. That means that if you modify these arguments inside the function, it will not affect the original values. This feature is known as sending arguments by value. Let's see an example:

function modify($a) {
    $a = 3;
}

$a = 2;
modify($a);
var_dump($a); // prints 2

We are declaring a variable $a with value 2, and then calling the modify method sending that $a. The modify method modifies the argument $a, setting its value to 3, but this does not affect the original value of $a, which remains 2 as you can see from var_dump.

If what you want is to actually change the value of the original variable used in the invocation, you need to pass the argument by reference. To do that, you add an ampersand (&) before the argument when declaring the function:

function modify(&$a) {
    $a = 3;
}

Now, on invoking the function modify, $a will always be 3.