PHP Default Parameters

Summary: In this tutorial, you’ll learn about PHP default parameters and how to use them to simplify function calls.

Introduction to the PHP default parameters #

The following defines the concat() function that concatenates two strings with a delimiter:

<?php

function concat($str1, $str2, $delimiter)
{
    return $str1 . $delimiter . $str2;
}Code language: HTML, XML (xml)

When you call the concat() function, you need to pass exactly three arguments. For example:

<?php

function concat($str1, $str2, $delimiter)
{
    return $str1 . $delimiter . $str2;
}

$message = concat('Hi', 'there!', ' ');

echo $message;Code language: HTML, XML (xml)

Try it

However, you’ll find that you often use the space ‘ ‘ as the delimiter. And it’s repetitive to pass the space whenever you call the function.

This is why default parameters come into play.

PHP allows you to specify a default argument for a parameter. For example:

<?php

function concat($str1, $str2, $delimiter = ' ')
{
    return $str1 . $delimiter . $str2;
}Code language: HTML, XML (xml)

In this example, the $delimiter parameter takes the space as the default argument.

When you call the concat() function and don’t pass the delimiter argument, the function will use the space for the $delimiter like this:

<?php

function concat($str1, $str2, $delimiter = ' ')
{
    return $str1 . $delimiter . $str2;
}

$message = concat('Hi', 'there!');

echo $message;Code language: HTML, XML (xml)

Try it

Output:

Hi there

However, if you pass an argument for the $delimiter, the function will use that argument instead:

<?php

function concat($str1, $str2, $delimiter = ' ')
{
    return $str1 . $delimiter . $str2;
}

$message = concat('Hi', 'there!', ',');

echo $message;Code language: HTML, XML (xml)

Try it

Output:

Hi,there!

In this example, we passed a comma to the $delimiter. The concat() function used the comma (,) instead of the default argument.

When you specify a default argument for a parameter, it becomes optional. This means that you can pass a value or skip it.

Default arguments #

The default arguments must be constant expressions. They cannot be variables or function calls.

PHP allows you to use a scalar value, an array, and null as the default arguments.

The order of default parameters #

When you use default parameters, it’s a good practice to place them after the parameters that don’t have default values. Otherwise, you will get unexpected behavior. For example:

<?php

function concat($delimiter = ' ', $str1, $str2)
{
	return $str1 . $delimiter . $str2;
}

$message = concat('Hi', 'there!', ',');

echo $message;
Code language: HTML, XML (xml)

Try it

Output:

there!Hi,

Summary #

• Use default parameters to simplify the function calls.
• Default parameters are optional.