Functions

PHP comes with hundreds of built in functions, to sort arrays, find the length of strings, match regexes, or even get the timestamp corresponding to midnight on Easter of a given year, but you will inevitably want to define your own functions. Function definitions look like this:

function getGreeting(string $planetName): string
{
    return "Hello, $planetName.\n";
}

Notice the type declarations, string and : string. Strictly speaking these are optional, but you should add them whenever you can. PHP is a gradually typed language, like TypeScript. However unlike Typescript, which does type checking in the compiler, PHP applies type checking only when the relevant part of your program runs. Let's see an example:

<?php declare(strict_types=1);

function getGreeting(string $planetName): string
{
    return "Hello, $planetName.\n";
}

echo getGreeting('Magrathea');
echo getGreeting(42);

Running this, you should see something like:

Hello, Magrathea.
PHP Fatal error:  Uncaught TypeError: Argument 1 passed to getGreeting() must be of the type string, int given, called in functions.php on line 9 and defined in functions.php:3
Stack trace:
#0 functions.php(9): getGreeting()
#1 {main}
  thrown in functions.php on line 3

The first part of the program ran fine, but attempting to pass an integer to a function that we've declared with a string parameter was a fatal error and caused a crash. If we'd tried to return anything other than a string from inside the function that would also be a fatal error.