PHP Variables

A variable in PHP is a name of memory location that holds data. In PHP Variables are represented by a dollar sign followed by the name of the variable. The variable name is case-sensitive.

A variable can have a short name (like a and a) or a more descriptive name (name, class, section).




Rules for PHP variables:

1. A variable starts with the $ sign, followed by the name of the variable
2. A variable names are case-sensitive ($age and $AGE are two different variables)
3. A variable name must start with a letter or the underscore character
4. A variable name cannot start with a number
5. A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )

Declaring PHP Variables :

In php we can store string, int, float values in variables.

<?php  
$str="hello string variable";  //String type
$x=100;  // integer type
$y=10.6;  // float type
echo "string is: $str <br/>";  
echo "integer is: $x <br/>";  
echo "float is: $y <br/>";  
?>

Output :

-----------------------------------

string is: hello string variable

integer is: 100

float is: 10.6 

Sum of two integer variables :

<?php
$x = 5;
$y = 4;
echo $x + $y;
?>

Output :

----------------------

9

Concatenate two string variables :

<?php
   $a = "first variable";
   $b = 'second variable';
   
   echo $a." ".$b ;
?>

Output :

first variable second variable

Best part of php variable is automatically converts the variable to the correct data type, depending on its value.

PHP Variables Scope :

In PHP, variables can be declared anywhere in the script.
The scope of a variable is the part of the script where the variable can be referenced/used.
PHP has three different variable scopes:

1. local
2. global
3. static

Global Scope :

• A variable declared outside a function is called as GLOBAL SCOPE 
• It can be only be accessed outside a function.

Example : 1 (Can't access global scope inside function)

<?php
$x = 5; // global scope

function myTest() {
  //below line of code throw error message.
  //we can access the global variable inside function
   echo "access global variable $x ";
} 
myTest();

echo "Variable x outside function is: $x";
?>

Output:
------------------------------
access global variable Variable x outside function is: 5PHP Notice: Undefined variable: x in /web/com/1501411108_9799/main.php on line 7

Example : 2 (Global scope only accessible outside function)

<?php
$x = 5; // global scope

function myTest() {
  //below line of code throw error message.
  //we can access the global variable inside function
   echo "local scope <br>";
} 
myTest();

echo "Variable x outside function is: $x";
?>

Output :
---------------------
local scope
Variable x outside function is: 5

Global Keyword :
The global keyword is used to access a global variable from within a function. Let see the below example.

<?php
$x = 5;
$y = 10;

function add() {
    global $x, $y;
    echo "$x<br>"; // output 5
    echo "$y<br>"; // output 10
    $y = $x + $y;
}

add();
echo $y; // output 15
?>

Output :
-----------------------
5
10
15

Local Scope :

• A variable declared within a function is called as LOCAL SCOPE.
• It can only be accessible within that function.

Example :1 (Throw error message while accessing local variable outside function)

<?php
function demo() {
    $x = 5; // local scope
    echo "access inside the function: $x";
} 
demo();

// using x outside the function will generate an error
echo "can't access outside function : $x";
?>

Output :
---------------------------------
access inside the function: 5can't access outside function : PHP Notice: Undefined variable: x in /web/com/1501411108_9799/main.php on line 9

Example : 2 ( Access local variable inside function)

<?php
function demo() {
    $x = 5; // local scope
    echo "access inside the function: $x";
} 
demo();
?>

Output :
------------------------------
access inside the function: 5

Static Keyword:
Normally, when a function is completed/executed, all of its variables are deleted. However, sometimes we want a local variable NOT to be deleted. In that case we use Static keyword.

<?php
   function keep_track() {
      STATIC $count = 0;
      $count++;
      print $count;
      print "<br />";
   }
   
   keep_track();
   keep_track();
   keep_track();
?>

Output:
----------------------
1
2
3

Const keyword :
constants defined using the const keyword must be declared at the top-level scope because they are defined at compile-time. This means that they cannot be declared inside functions, loops, if statements or try/ catch blocks.
Constant is case sensitive.
Constant can not be changed.

<?php  
const MESSAGE="Hello const by JavaTpoint PHP";  
echo MESSAGE;  
?>

Output :
Hello const by JavaTpoint PHP

Like other programming language php variable support different kind of data types, which are as follows:

1. String
2. Integer
3. Float (floating point numbers - also called double)
4. Boolean
5. Array
6. Object
7. NULL