phpapprentice/code/functions.php

<?php

// A function allows you to store code under a name and then execute
// that code later.

// A function always starts with the
// function keyword followed by the name with parentheses and then
// opening and closing curly braces around the code.
function hello_world() {
    echo "hello world\n";
}

// To call the function, use the function name with parentheses.
hello_world();

// You can set up values to be passed into a function.
// To do so, write variables in between the function parentheses.
// Each one should be separated by a comma.
function greet($firstname, $lastname) {
    echo "hello $firstname $lastname\n";
}

// Then, you can pass in values when calling a function. In the greet function,
// 'John' is assigned to $firstname and 'Smith' is assigned to
// $lastname.
greet('John', 'Smith');

// You can also return a value from a function. You can only
// return a single value from a function.
function capitalize($value) {
    return strtoupper($value);
}

// When calling a function, it will output the return value which
// you can load into a variable.
$animal = capitalize('dog');
echo "$animal\n";

// You can also create nameless functions called closures. Closures can be
// stored in variables or passed into other functions.
$sum = function ($a, $b) {
    return $a + $b;
};

// You can execute a closure by putting parentheses after the variable.
echo $sum(1, 2) . "\n";