PHP Variable Functions

Summary: in this tutorial, you will learn about the PHP variable functions and how to use them to call a function, a method of an object, and a class’s static method.

Introduction to PHP variable functions

Variable functions allow you to use a variable like a function. When you append parentheses () to a variable, PHP will look for the function whose name is the same as the value of the variable and execute it. For example:

<?php $f = 'strlen'; echo $f('Hello');
Code language: HTML, XML (xml)



How it works.

  • First, define a variable $f and initialize its value to the 'strlen' literal string.
  • Second, use the $f as a function by passing the string 'Hello' to it.

When PHP sees $f(), it looks for the strlen() function. Because the strlen() is a built-in function, PHP just invokes it.

If PHP cannot find the function name, it’ll raise an error. For example:

<?php $f = 'len'; echo $f('Hello');
Code language: HTML, XML (xml)


Fatal error: Uncaught Error: Call to undefined function len() in index.php:5
Code language: plaintext (plaintext)

In this example, because PHP cannot find the len() function, it issues an error.

More variable function examples

Let’s take some examples of using the variable functions.

1) Using variable functions to call a method example

The variable functions allow you to call the methods of an object. The syntax for calling a method using a variable function is as follows:

Code language: PHP (php)

Notice that you need to prefix the variable name with the $ sign. In this case, you’ll have the $ sign before the this keyword and the variable name. For example:

<?php class Str { private $s; public function __construct(string $s) { $this->s = $s; } public function lower() { return mb_strtolower($this->s, 'UTF-8'); } public function upper() { return mb_strtoupper($this->s, 'UTF-8'); } public function title() { return mb_convert_case($this->s, MB_CASE_TITLE, 'UTF-8'); } public function convert(string $format) { if (!in_array($format, ['lower', 'upper', 'title'])) { throw new Exception('The format is not supported.'); } return $this->$format(); } }
Code language: HTML, XML (xml)

How it works:

  • First, define a Str class that has three methods for converting a string to lowercase, uppercase, and title case.
  • Second, define the convert() method that accepts a string. If the format argument is not one of the method names: lower, upper, and title, the convert() method will raise an exception. Otherwise, it’ll call the corresponding method lower(), upper() or title().

The following shows how to use the convert() method of the Str class:

<?php require_once 'Str.php'; $str = new Str('Hello there'); echo $str->convert('title');
Code language: HTML, XML (xml)


Hello There

2) Using variable functions to call a static method example

The following example uses a variable function to call a static method:

<?php class Str { private $s; public function __construct(string $s) { $this->s = $s; } public function __toString() { return $this->s; } public static function compare(Str $s1, Str $s2) { return strcmp($s1, $s2); } }
Code language: HTML, XML (xml)

The Str class has a constructor that accepts a string. It implements the toString() method that converts the Str instance to a string.

The Str class has the compare() static method that compares two instances of the Str class. To call the compare() static method using a variable function, you use the following:

$str1 = new Str('Hi'); $str2 = new Str('Hi'); $action = 'compare'; echo Str::$action($str1, $str2); // 0
Code language: PHP (php)


  • Append parentheses () to a variable name to call the function whose name is the same as the variable’s value.
  • Use the $this->$variable() to call a method of a class.
  • Use the className::$variable() to call a static method of a class.
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