33 PHP Arrays, Functions, and Classes

PHP contains support for arrays, functions, and classes. Since you are already familiar with these programming constructs from your previous experience in JavaScript and other languages, this section gives you the basics you need to start using them in PHP as well.

Note that in this book, we mostly focus on short, single-use-case scripts written in PHP. You will need to use PHP arrays in some cases, but you may not ever need functions or classes so you could safely skip those sections if you are in a hurry.

PHP Arrays

PHP arrays are a lot like JavaScript arrays. They can contain elements of different types, and they have no fixed length. PHP also supports array literals using square brackets.

$a = ["red", 23, 55.7];

You can access and iterate through array elements by index, using the global count function to access the number of items in the array.

for ($i = 0; $i < count($a); $i++) {
    echo $a[$i];
}

PHP also has a foreach loop, which is similar to JavaScript, but has a slightly different syntax (the array variable comes first, followed by the variable that will hold each element of the array):

foreach ($a as $element) {
    echo $element;
}

You can embed array references into double-quoted strings:

echo "Element $i is $a[$i].";

Arrays are not objects in PHP, so there are no array methods. All helper functions for arrays are global functions. Two that are of particular use are count($a), which returns the length of $a and array_push($a, $item), which adds $item to the end of the array $a.

An Aside on Strings

Strings in PHP are just arrays of characters. You can access them as if they were arrays. But there is no char type in PHP, so if $s is a string, $s[4] returns a string containing the 5th character of $s.  The script below will capitalize all the “s” characters in string $s.

$s = "server-side scripting is fun!";
for ($i = 0; $i < strlen($s); $i++) {
    if ($s[$i] === "s") {
        $s[$i] = "S";
    }
}

Strings are not objects in PHP, so there are no string methods or instance variables. The code above gets the length of the string by using the global strlen() function.

Functions

Functions are declared with the function keyword, just like in JavaScript. They are not quite as flexible as JavaScript functions. If a function has 3 parameters, you must call it with 3 arguments. If a function does not execute a return statement, it will return the value null by default.

Here’s a simple example of a function.

function spam($a, $b, $c) {
    return $a + $b + $c;
}

You could call it like this:

<p> <?= spam(1,2,3) ?> </p>

As with JavaScript, you can also define default argument values, as shown below:

function spam($a, $b=0, $c=0) {
    return $a + $b + $c;
}

With the declaration above, all the following calls to spam are legal:

<p>
    <?= spam(1) ?>
    <?= spam(1, 2) ?>
    <?= spam(1, 2, 3) ?>
</p>

Classes

Full support for object-oriented programming using classes was added to PHP in version 5. Before that, PHP was a purely imperative language. Now you can program using a mix of the two styles. If you want to define your own classes in PHP, the syntax is not that different from what you learned in JavaScript Classes and Objects.

Here’s an example of a class declaration in PHP:

class Person {
    // instance variables (optional)
    public $name = "";
    public $age = -1;

    // constructor is named __construct (two underscores)
    // $this->name references the name variable
    function __construct($name, $age = -1) {
        $this->name = $name;
        $this->age = $age;
    }

    // method to increase the $age instance variable
    function increase_age($increment) {
        $this->age += $increment;
    }

    // returns an HTML string to display this object
    function view() {
        return "<p>$this->name: $this->age years old</p>";
    }
}

$p1->$name
$this->$name

In the above example, the declaration of instance variables outside of the constructor is optional.

Here’s how you create objects of type Person.

$p1 = new Person("Sam");         
$p2 = new Person("Anne", 30);

Here’s how you access a method.

$p1->increase_age(11);

Note the use of the arrow operator (->) instead of the dot operator. The dot operator is reserved for string concatenation in PHP.

You can include instance variable references inside a double-quoted string.

echo "Your name is $p1->name.";

In the above example, you could also use the view method, but you need braces { } to include a method call inside a double-quoted string.

echo "<div>{$p2->view()}</div>";

include "person.php";