Variables are containers for storing data such as strings, integers, boolean values, array and objects.

PHP follows certain rules for variable declarations such as:

• The variable must begin with a dollar sign ($)

  Example:

  <?php
   $var = 5;
  ?>

• The variable name can contain characters such as A-Z, a-z, 0-9, _ and ASCII characters from 127-255.

  Example:

  <?php
  $var = 5; //Valid
  $var_1 = "Foo"; //Valid
  $_var2 = 'Bar'; //Valid
   $var.3 = 'Baz'; //Invalid
  ?>

• The variable name can begin with underscore (_).

  Example:

  <?php
    $_var2 = 'Bar'; //Valid
  ?>

• The variable name must not begin with a number 0-9.

  Example:

  <?php
    $9var3 = 'Baz'; //Invalid
  ?>

• The variable name is case sensitive.

  Example:

<?php
    $var = 5; //Valid
    $VAR = "Foo"; //Valid
    echo $var; //Output 5
    echo "<br>";
    echo $VAR; //Output Foo
?>

PHP is a loosely typed language, hence we don't need to declare the data type of a variable when declaring the variable. Unlike Java or C.

<?php
    $var = 5; 
    $var2 = 4; 
    $sum = $var+$var2;
    echo $sum; //Output 9
    echo "<br>";
    echo $var+$var2; //Output 9
?>

The variables can also be assigned by referencing. This allows two variables to refer to the same content. The & operator is placed before the variable that is to be referenced.

Example :

<?php
    $var1 = "foo";
    $var2 = "bar";

myTest($var1, $var2);

echo $var1; //Output foo
echo $var2; //Output BAR

function myTest($var1, &$var2){
    $var1 = "FOO";
    $var2 = "BAR";
}
?>

To have variable names set dynamically we use the variable variables. This can be particularly useful when there is a need to create multiple variables.

Example :

<?php

    $var = 'Tom'; 
    echo $var;      //Output Tom
    $$var = 'Cat'; //The value of $$var is the value of $var. So $$var and $Tom give the same output.
    echo $$var;   //Output Cat
    echo $Tom;   //Output Cat
?>

Scope of variable refers to the places from where a variable is accessible.

• Global scope is for the variables that are declared outside a function. These variables can be accessed from anywhere but not within a function.
• Local scope is for the variables declared within a function that can not be accessed from anywhere outside the function.

Example:

<?php
   $global = "Hello";

  function Test(){
   $local = "World";
   echo $global; //Error
   echo $local; //Output World
  }

   Test();
   echo $global; //Output Hello
   echo $local; //Error
?>

To access global variables inside a function:

<?php
   $global = "Hello";

 function Test(){
   global $global;
   $local = "World";
   echo $global; //Output Hello
   echo $local; //Output World
  }

   Test();
   echo $global; //Output Hello
   echo $local; //Error
?>

Everytime a function is created all of its local variables are deleted. To retain the variable's last value we declare it static.

Example :

<?php

 function WithStatic(){
   static $var = 0;
   echo $var;
   $var++;
  }

   WithStatic(); //Output 0
   WithStatic(); //Output 1
   WithStatic(); //Output 2

 function WithoutStatic(){
   $var = 0;
   echo $var;
   $var++;
  }

   WithoutStatic(); //Output 0
   WithoutStatic(); //Output 0
   WithoutStatic(); //Output 0
?>