Why is type hinting necessary in PHP?

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Type hinting isn’t required, but it can allow you to catch certain types of mistakes. For example, you might have a function or method which requires an integer. PHP will happily convert “number looking strings” into integers, and this can cause hard to debug behaviour. If you specify in your code that you specifically need an integer, this can prevent those kinds of bugs in the first place. Many programmers consider protecting their code in this way to be a best practice.

As a concrete example of this in action, let’s look at an updated version of your index.php file:

index.php

<?php
include 'Song.php';
include 'Test.php';

$song_object = new Song;
$test_object = new Test;

$song_object->title = "Beat it!";
$song_object->lyrics = "It doesn't matter who's wrong or right... just beat it!";

$test_object->title = "Test it!";
$test_object->lyrics = "It doesn't matter who's wrong or right... just test it!";


function sing(Song $song)
{
    echo "Singing the song called " . $song->title;
    echo "<p>" . $song->lyrics . "</p>";
}

sing($song_object);
sing($test_object);

As well as the new Test.php file I added:

Test.php

<?php

class Test
{
    public $title;
    public $lyrics;
}

When I run index.php now, I get the following error:

Output:

Singing the song called Beat it!<p>It doesn't matter who's wrong or right...
just beat it!</p>PHP Catchable fatal error:  Argument 1 passed to sing() must
be an instance of Song, instance of Test given, called in test/index.php on
line 22 and defined in test/index.php on line 15

Catchable fatal error: Argument 1 passed to sing() must be an instance of
Song, instance of Test given, called in test/index.php on line 22 and defined
in test/index.php on line 15

This is PHP letting me know that I tried to use the wrong type of class when I called the sing() function.

This is useful because, even though the above example worked, the Test class could be different than the Song class. This could lead to hard to debug errors later on. Using hinting in this way gives developers a way to prevent type errors before they cause issues. This is especially useful in a language like PHP which is often eager to auto-convert between types.

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