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chapters/functions.md

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+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.
+```php
+<?php
+
+function hello_world() {
+    echo "hello world\n";
+}
+```
+
+To call the function, use the function name with parentheses.
+```php
+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.
+```php
+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.
+```php
+greet('John', 'Smith');
+```
+
+You can also return a value from a function. You can only
+return a single value from a function.
+```php
+function capitalize($value) {
+    return strtoupper($value);
+}
+```
+
+When calling a function, it will output the return value which
+you can load into a variable.
+```php
+$animal = capitalize('dog');
+echo "$animal\n";
+```
+
+You can also create nameless functions called closures. Closures can be
+stored in variables or passed into other functions.
+```php
+$sum = function ($a, $b) {
+    return $a + $b;
+};
+```
+
+You can execute a closure by putting parentheses after the variable.
+```php
+echo $sum(1, 2) . "\n";
+```