# PHP Programming/Functions
## Introduction

Functions (or methods in the context of a class/object) are a way to group common tasks or calculations to be re-used simply.
Functions in computer programming are much like mathematical functions: You can give the function values to work with and get a result without having to do any calculations yourself.
You can also find a huge list of predefined functions built into PHP in the PHP Manual's function reference.

## How to call a function

Note that echo is not a function. "Calling a function" means causing a particular function to run at a particular point in the script. The basic ways to call a function include:
* calling the function to write on a new line (such as after a ";" or "}")
* calling the function to write on a new line inside a control
* assigning the returned value of a function to a variable "$var = function"
* calling a function inside the argument parentheses (expression) of a control
In our earlier examples we have called several functions. Most commonly we have called the function print to print text to the output. The parameter for echo has been the string we wanted printed (for example print("Hello World!") prints "Hello World!" to the output).
If the function returns some information, we assign it to a variable with a simple assignment operator "=":

## Parameters

Parameters are variables that exist only within that function. They are provided by the programmer when the function is called and the function can read and change them locally (except for reference type variables that are changed globally, which is a more advanced topic).
When declaring or calling a function that has more than one parameter, you need to separate between different parameters with a comma ','.
A function declaration can look like this:
To call this function, you must give the parameters a value. It doesn't matter what the value is, as long as there is one:
Output:
When declaring a function, you sometimes want to have the freedom not to use all the parameters. Therefore, PHP allows you to give them default values when declaring the function:
These values will only be used, if the function call does not include enough parameters. If there is only one parameter provided, then $var2 = "I'm Learning PHP":
Output:
Another way to have a dynamic number of parameters is to use PHP's built-in func_num_args, func_get_args, and func_get_arg functions.
or
The above functions would calculate the arithmetic mean of all of the values passed to them and output it. The difference is that the first function uses func_num_args and func_get_arg, while the second uses func_get_args to load the parameters into an array. The output for both of them would be the same. For example:
Output:

## Returning a value

This function is all well and good, but usually you will want your function to return some information. Generally there are two reasons why a programmer would want information from a function:
* The function does tasks such as calculations, and we need the result.
* A function can return a value to indicate, if the function encountered any errors.
To return a value from a function use the return statement in the function.
Example PHP script:
Result:
Notice that a return statement ends the function's course. If anything appears in a function declaration after the return statement is executed, it is parsed but not executed. This can come in handy in some cases. For example:
Notice that there is no else after the if. This is due to the fact that, if $divider does equal 0, the return statement is executed and the function stops.
If you want to return multiple variables, you need to return an array rather than a single variable. For example:
When calling this from your script, you need to call it into an array. For example:
In this case $return['tot'] will be the total (e.g. 15), while $return['diff'] will be the difference (5).

## Runtime function usage

A developer can create functions inside a PHP script without having to use the function name($param...) syntax. This can be done by way of programming that can let you run functions dynamically.

### Executing a function that is based on a variable's name

There are two ways to do it: either using the direct call, or the call_user_func or the call_user_func_array:

#### Using call_user_func* functions to call functions

call_user_func and call_user_func_array only differ in that the call_user_func_array allows you to use the second parameter as array to pass the data very easily, and call_user_func has an infinite number of parameters that is not very useful in a professional way. In these examples, a class will be used for a wider range of using the example:
Using call_user_func:
Using call_user_func_array:
Note how call_user_func and call_user_func_array are used in both of the examples. call_user_func_array allows the script to execute the function more dynamically.
As there was no example of using both of these functions for a non-class function, here they are:
Using call_user_func:
Using call_user_func_array:

#### More complicated examples
### Creating runtime functions

Creating runtime functions is a very good way of making the script more dynamic:
create_function creates a function with parameters $one and $two, with a code to evaluate return… When create_function is executed, it stores the function's info in the memory and returns the function's name. This means that you cannot customise the name of the function although that would be preferred by most developers.

### Citations

* ↑ echo