PHP Functions

PHP Built-in Functions

PHP comes with more than 1000 built-in functions, each doing a specific job, and you can call any of them directly from your script. For the full list, see our PHP reference.

PHP User Defined Functions

You aren't limited to what PHP provides, because you can also write functions of your own.

  • Think of a function as a group of statements you can reuse throughout a program.
  • Loading a page doesn't run it on its own.
  • Its code runs only when you call it.

Create a Function

To declare your own function, write the keyword function and then the name you want to give it.

The function's code goes between a pair of curly braces: { marks where it begins and } marks where it ends.

Note: Function names must begin with a letter or an underscore, and PHP ignores their letter case.

Tip: Choose a name that says what the function does, so your code is easier to read.

Function Syntax

The general shape of a function you define yourself:

index.php 
function functionName($parameter1, $parameter2) {
  // code to be executed
  return $value; // optional
}

Create a Function Example

Here is a real one, called myMessage(), that prints a greeting:

index.php 
function myMessage() {
  echo "Hello world!";
}

Call a Function

Calling a function takes nothing more than its name and a pair of parentheses ().

When called, this one prints "Hello world!":

Call a Function

First define myMessage(), then run it by calling it:

Prompt
Write a PHP script that defines a function named myMessage which prints "Hello world!", then calls the function.
Output: PHP script

PHP Function Parameters

Parameters are how you hand information to a function. Inside the function, a parameter behaves just like a variable.

List them inside the parentheses after the function's name, with a comma between each one. There is no limit on how many you can add.

Function With One Parameter

familyName() takes a single parameter, $fname. Each call passes in a different first name, which the function combines with the same last name, so every line of output shows a new first name with "Refsnes" after it:

Prompt
Write a PHP script that defines a function named familyName with one parameter for the first name, and prints the first name followed by "Refsnes." for five different names.
Output: PHP script

Function With Two Parameters

This version takes two parameters, $fname and $year, and prints both:

Prompt
Write a PHP script that defines a function named familyName with two parameters for first name and birth year, and prints a message with both values for three different people.
Output: PHP script

PHP Default Parameter Value

A parameter can have a fallback value. Here, calling setHeight() with no argument means it uses its default instead:

Default Parameter Value

The first call passes 350; the second passes nothing, so $height falls back to 50:

Prompt
Write a PHP script that defines a function named setHeight with a default parameter value of 50, then calls it with and without an argument.
Output: PHP script

PHP Functions - Returning values

When a function reaches return, it stops right there and sends a value back to the line of code that called it:

Returning Values

sum() adds its two arguments and gives the total back to the caller:

Prompt
Write a PHP script that defines a function named sum that takes two numbers, returns their sum, and demonstrates it with three different pairs.
Output: PHP script

Passing Arguments by Reference

By default, arguments are passed by value: the function works on a copy, so the original variable you passed in stays unchanged.

Passing by reference removes that copy, which means anything the function does to the argument also happens to the original variable. To set this up, put the & operator in front of the parameter:

Pass by Reference

Because $value is a reference, add_five() changes $num itself, from 2 to 7:

Prompt
Write a PHP script that defines a function named add_five that takes a parameter by reference and adds 5 to it, then prints the updated value.
Output: PHP script

Variable Number of Parameters

Sometimes you can't know in advance how many values a function will receive. Put the ... operator in front of a parameter and it will accept any number of them. A function like this is called a variadic function.

All the values collected by that parameter arrive as an array.

There are two rules: a function can have only one variable-length parameter, and it must come last. Put it anywhere else and PHP reports an error.

Variadic Function

sumMyNumbers() adds up however many numbers it is given, in this case six:

Prompt
Write a PHP script that defines a variadic function named sumMyNumbers that accepts any number of arguments and returns their sum.
Output: PHP script

Variadic as Last Argument

A normal parameter can come first, as long as the variadic one is in the last position:

Prompt
Write a PHP script that defines a function named myFamily with a fixed last name parameter and a variadic first names parameter, then prints a greeting for each family member.
Output: PHP script

Variadic Not Last Argument Error

Here ... is on the first of two parameters, so the code fails with an error:

Prompt
Write a PHP script that incorrectly places the variadic parameter before a regular parameter to demonstrate the resulting error.
Output: PHP script

PHP is a Loosely Typed Language

You may have noticed that none of the examples so far told PHP what data type a variable holds.

That's because PHP works out the type for itself from the variable's value. With no types enforced, you can do things like add a numeric string such as "5" to an integer, and no error occurs.

Type declarations arrived in PHP 7. They let you state which data type you expect when you declare a function. Turn on the strict declaration as well, and a mismatched type stops the script with a "Fatal Error".

You enable strict mode with declare(strict_types=1);, which has to be the very first statement in the PHP file.

Strict Mode

The strict declaration is on here, and the function receives a number and a string. Because "5 days" isn't an integer, the script ends with an error:

Prompt
Write a PHP script with strict_types enabled that defines a function addNumbers with int type hints, and calls it with a string to demonstrate the error.
Output: PHP script

PHP Return Type Declarations

Type declarations work for the return statement too.

To set the type a function gives back, write a colon : and the type just before the opening curly { bracket in the declaration.

Return Type Declaration

This function declares float as its return type:

Prompt
Write a PHP script with strict_types enabled that defines a function addNumbers with float parameters and a float return type declaration, then calls it.
Output: PHP script

Different Return Type

The return type doesn't have to match the argument types, as long as the value you return really is of the declared type. Here, the float result is cast to an int:

Prompt
Write a PHP script with strict_types enabled that defines a function addNumbers with float parameters and an int return type, casting the result to int.
Output: PHP script