PHP Default Parameters

Summary: in this tutorial, you’ll learn about PHP default parameters and how to default parameters to simplify the 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)

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)

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)

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, the parameter becomes optional. It 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)

Output:

there!Hi,

Summary

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