The real power of PHP comes from its functions.
PHP has more than 1000 built-in functions, and in addition you can create your own custom functions.
PHP Built-in Functions
PHP has over 1000 built-in functions that can be called directly, from within a script, to perform a specific task.
Please check out our PHP reference for a complete overview of the PHP built-in functions.
PHP User Defined Functions
Besides the built-in PHP functions, it is possible to create your own functions.
- A function is a block of statements that can be used repeatedly in a program.
- A function will not execute automatically when a page loads.
- A function will be executed by a call to the function.
Create a User Defined Function in PHP
A user-defined function declaration starts with the word function:
Syntax
function functionName[] {
code to be executed;
}
Note: A function name must start with a letter or an underscore. Function names are NOT case-sensitive.
Tip: Give the function a name that reflects what the function does!
In the example below, we create a function named "writeMsg[]". The opening curly brace [ { ] indicates the beginning of the function code, and the closing curly brace [ } ] indicates the end of the function. The function outputs "Hello world!". To call the function, just write its name followed by brackets []:
Example
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PHP Function Arguments
Information can be passed to functions through arguments. An argument is just like a variable.
Arguments are specified after the function name, inside the parentheses. You can add as many arguments as you want, just separate them with a comma.
The following example has a function with one argument [$fname]. When the familyName[] function is called, we also pass along a name [e.g. Jani], and the name is used inside the function, which outputs several different first names, but an equal last name:
Example
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The following example has a function with two arguments [$fname and $year]:
Example
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PHP is a Loosely Typed Language
In the example above, notice that we did not have to tell PHP which data type the variable is.
PHP automatically associates a data type to the variable, depending on its value. Since the data types are not set in a strict sense, you can do things like adding a string to an integer without causing an error.
In PHP 7, type declarations were added. This gives us an option to specify the expected data type when declaring a function, and by adding the strict declaration, it will throw a "Fatal Error" if the data type mismatches.
In the following example we try to send both a number
and a string to the function without using strict:
Example
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To specify strict we need to set declare[strict_types=1];. This must be on
the very first line of the PHP file.
In the following example we try to send both a number and a string to the function, but here we have added the strict declaration:
Example
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The strict declaration forces things to be used in the intended way.
PHP Default Argument Value
The following example shows how to use a default parameter. If we call the function setHeight[] without arguments it takes the default value as argument:
Example
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PHP Functions - Returning values
To let a function return a value, use the return statement:
Example
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PHP Return Type Declarations
PHP 7 also supports Type Declarations
for the return statement. Like with the type declaration for function arguments, by enabling the strict requirement, it will throw a "Fatal Error" on a type mismatch.
To declare a type for the function return, add a colon [ : ] and the type right before the opening curly [ { ]bracket when declaring the function.
In the following example we specify the return type for the function:
Example
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You can specify a different return type, than the argument types, but make sure the return is the correct type:
Example
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Passing Arguments by Reference
In PHP, arguments are usually passed by value, which means that a copy of the value is used in the function and the variable that was passed into the function cannot be changed.
When a
function argument is passed by reference, changes to the argument also change the variable that was passed in. To turn a function argument into a reference, the & operator is used:
Example
Use a pass-by-reference argument to update a variable:
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