How to use PHP functions to create reusable code blocks

PHP functions enable you to invoke identical code blocks multiple times without duplication, streamlining tasks and minimising error potential.

What are PHP functions?

PHP functions are named blocks of code that execute a specific or a series of instructions. Vital in structured programming, they break PHP code into reusable, organised units. PHP provides built-in functions for common tasks like string, array handling, and PHP loops, while user-defined functions let you tailor processes in your applications.

What is the difference between internal and user-defined PHP functions?

Internal functions are already built into programming languages. Many are an integral part of a language library and are available without further steps. Others require special extensions to be installed. In PHP, these functions efficiently handle common tasks, such as the strlen() function, which calculates string length. They are typically optimised for high performance.

In contrast, user-defined functions are self-written code sections. They require explicit definition within the code. These PHP functions are commonly tailored for specific purposes or problem-solving within software projects. User-defined functions may encompass intricate algorithms or data manipulation. For instance, consider custom functions that utilise PHP to fetch data from a MySQL database and format it for web page display.


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How PHP functions are structured

User-defined PHP functions follow a certain pattern. The basic structure is:

function functionName(parameter1, parameter2, ...) {
    // php function example code block
    return result; // optional

Here is an explanation of the different parts of a PHP function:

  • function: This keyword signals the beginning of the function definition.
  • functionName: The name of the function. It should be unique and describe the purpose of the function.
  • parameter1, parameter2, …: Parameters are values that are passed to the PHP function when it’s called. They’re optional, and you can use as many as you need.
  • code block: Code executes a task or performs the desired operations.
  • return result: This is optional. If the function is to return a result, use the return statement.

Internal functions don’t need to be defined. You can call them directly via their identifier.


For newcomers to PHP programming, we recommend our PHP tutorial. If you’re not sure whether PHP is the right programming language for you, take a look at our comparisons PHP vs. Python and PHP vs. JavaScript.


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Examples for the use of PHP functions

In the following, we present practical applications of PHP functions.

Assign default values to parameters

You can set default values for PHP function parameters by specifying the desired default value in the function definition. If you don’t provide a value for this parameter when calling the function, the default will be used.

function greet($name = "Tim") {
    echo "Hello, $name";
greet(); // Output: "Hello, Tim"
greet("Max"); // Output: "Hello, Max"

Pass arguments by reference

Values are transferred to a function either ‘by value’ or ‘by reference’. ‘By value’ means that a copy of the value is used and changes have no effect on the original value outside the function. With ‘by reference’, the actual variable is passed to the function and modifications within the function affect the original value. This is done by prefixing the & symbol in the function definition.

function incrementByOne(&$num) {
$val = 5;
echo $val; // Output: "6"

Here we define the function incrementByOne with the parameter $num, passed by reference. The post-increment belongs to the PHP operators and increments numbers or character strings by 1. If you call the PHP function, the value of $val increases from 5 to 6. This change occurs externally because of the reference parameter passing.

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