# PHP Functions

A function is a block of code set with an identifier, that can to be executed at some point in the future. PHP has many predefined functions, like is_array(), that allow us to perform a specific task, whenever we need for as many times as we need. That is advantage of using functions. It creates reusable code.

# Calling a function

A function will not execute it's code until it is called. To call a function, write the function's name followed by a set of parentheses.

<?php phpinfo();

# Internal Functions

Internal Functions, as known as built-in functions, are function that come standard with PHP. PHP has hundreds of the internal functions, including some for variables, strings, arrays, and math.

The following are some examples of using some internal functions.

# isset

The isset function is used to determine if a variable is declared and has a value other than NULL.

<?php 
$name = 'John'; 

echo isset($name); // true 
echo isset($fname); // false

# implode

The implode function will join array elements into a string.

<?php 
$colors = ['red', 'blue', 'green'];

echo implode(",", $colors); // red,blue,green

# in_array

The in_array function checks if a specified value exists in an array.

<?php 
$colors = ['red', 'blue', 'green'];

echo in_array('red', $colors); // true
echo in_array('black', $colors); // false

# rand

The rand function generates a random integer. An optional minimum and maximum number can be added to limit the pool of numbers.

<?php 
  // getting a random integer between 1 and 10 (inclusive)
  echo rand(1, 10); // possible response: 5

# User-Defined Function

A user-defined function is a custom function that we create. A function is defined using the function keyword, an identifier or name, a set of parentheses, and a set of curly braces. The block of code to be executed goes inside the curly braces.

<?php
  function say_hello() {
    echo "Hello World!";
  }
?>

# Arguments

Function arguments are data that is passed to a function through comma-separated list.

<?php 
  function say_hello_to($word) {
    echo "Hello {$word}!";
  }

  $name = "John Doe";
  say_hello_to($name); // Hello John Doe!
?>

Functions can be defined with more than one parameter, and therefore can accept more than one argument. When working with more than one parameter, the same number of arguments must be given, in the same order.

<?php
  function better_hello($greeting, $target, $punct) {
    echo "{$greeting} {$target}{$punct}<br>";
  }

  $name = "John Doe";
  better_hello("Greetings", $name, "!!!"); // Greetings John Doe!!! 
?>

# Default Arguments

Another feature of function arguments is providing a default value. Default function arguments allow for values to be predefined. When the value of an argument remains the same most of time, providing a predefined value can be beneficial and time-saving.

Default values are provided when the function is defined, and the syntax is similar to setting a value to a variable.

<?php 
  function paint($room = "office", $color = "red") {
    return "The color of the {$room} is {$color}.<br>"; 
  }
?>

When calling a function with default arguments, values can be omitted. But values still need to be provided in the correct order.

<?php 
  function paint($room = "office", $color = "red") {
    return "The color of the {$room} is {$color}.<br>"; 
  }

  // calling function using defaults
  echo paint(); // The color of the office is red.

  // calling function with provided values
  echo paint("bedroom", "blue"); // The color of the bedroom is blue.

  // calling function with null value
  echo paint("bedroom", null); // The color of the bedroom is .

  // calling function with only room
  echo paint("bedroom"); // The color of the bedroom is red.

  // calling function with only color
  echo paint("blue"); // The color of the blue is red.
?>

# Returning Values

Returning values from a function is a common and preferred practice. Values can be returned using a return statement.

The return statement immediately ends execution of the current function and returns its argument as the value of the function call.

<?php
  function add ($a, $b) {
    $sum = $a + $b;
    return $sum;
  }

  $result1 = add(3, 4);
  $result2 = add(5, $result1);
  echo $result2; // 12
?>

# Variable Scope

Variable scope refers to the context in which a variable was defined where it is visible.

PHP has two types of scope, global scope and local scope. Variables defined inside a function are in local scope and are only accessible inside the function. Variables defined outside of a function is in global scope and are accessible everywhere except inside of a function.

<?php
  $a = 1; // global scope

  function test() {
    return $a; // local scope
  }

  var_dump(test()); // NULL
?>

If a global variable needs to be used in the a function, the variable must be defined inside the function using the global keyword. This ensure access to the global variable instead of the local one.

<?php
  $a = 1; // global scope

  function test() {
    global $a; // global scope

    return $a; // global scope
  }

  var_dump(test()); // int(1)
?>