PHP | Variables

Variables

Variables in a program are used to store some values or data that can be used later in a program. The variables are also like containers that store character values, numeric values, memory addresses, and strings. PHP has its own way of declaring and storing variables. 
There are a few rules, that need to be followed and facts that need to be kept in mind while dealing with variables in PHP:  

• Any variables declared in PHP must begin with a dollar sign ($), followed by the variable name.
• A variable can have long descriptive names (like $factorial, $even_nos) or short names (like $n or $f or $x)
• A variable name can only contain alphanumeric characters and underscores (i.e., ‘a-z’, ‘A-Z’, ‘0-9, and ‘_’) in their name. Even it cannot start with a number.
• A constant is used as a variable for a simple value that cannot be changed. It is also case-sensitive.
• Assignment of variables is done with the assignment operator, “equal to (=)”. The variable names are on the left of equal and the expression or values are to the right of the assignment operator ‘=’.
• One must keep in mind that variable names in PHP names must start with a letter or underscore and no numbers.
• PHP is a loosely typed language, and we do not require to declare the data types of variables, rather PHP assumes it automatically by analyzing the values. The same happens while conversion. No variables are declared before they are used. It automatically converts types from one type to another whenever required.
• PHP variables are case-sensitive, i.e., $sum and $SUM are treated differently.

Data types used by PHP to declare or construct variables:

• Integers
• Doubles
• NULL
• Strings
• Booleans
• Arrays
• Objects
• Resources

Example: 

PHP

<?php

$num = 20;

// function to demonstrate use of global variable
function global_var()
{
    // we have to use global keyword before
    // the variable $num to access within
    // the function
    global $num;

    echo "Variable num inside function : $num \n";
}

global_var();

echo "Variable num outside function : $num \n";

?>

Variable Scopes

Scope of a variable is defined as its extent in a program within which it can be accessed, i.e. the scope of a variable is the portion of the program within which it is visible or can be accessed. 
Depending on the scopes, PHP has three variable scopes: 

• Local variables: The variables declared within a function are called local variables to that function and have their scope only in that particular function. In simple words, it cannot be accessed outside that function. Any declaration of a variable outside the function with the same name as that of the one within the function is a completely different variable. We will learn about functions in detail in later articles. For now, consider a function as a block of statements. 
  

Example: 

PHP

<?php

// function to demonstrate static variables
function static_var()
{
    // static variable
    static $num = 5;
    $sum = 2;

    $sum++;
    $num++;

    echo $num, "\n";
    echo $sum, "\n";
}

// first function call
static_var();

// second function call
static_var();

?>

Output: 

local num = 50 
Variable num outside local_var() is 60 

• Global variables: The variables declared outside a function are called global variables. These variables can be accessed directly outside a function. To get access within a function we need to use the “global” keyword before the variable to refer to the global variable.

Example: 

PHP

<?php
   $a = 'hello'; //hello is value of variable $a
   $$a = 'World'; //$($a) is equals to $(hello)
   echo $hello; //$hello is World i.e. $hello is new variable with value 'World'
?>

Output: 

Variable num inside function : 20 
Variable num outside function : 20 

Static variable: It is the characteristic of PHP to delete the variable, once it completes its execution and the memory is freed. But sometimes we need to store the variables even after the completion of function execution. To do this we use the static keywords and the variables are then called static variables.  PHP associates a data type depending on the value for the variable.

Example: 

Output: 

6
3
7
3

You must have noticed that $num regularly increments even after the first function call but $sum doesn’t. This is because $sum is not static, and its memory is freed after the execution of the first function call. 

Variable Variables:-

• PHP allows us to use dynamic variable names, called variable variables.
• Variable variables are simply variables whose names are dynamically created by another variable’s value.

Example:

PHP

<?php
   $a = 'hello'; //hello is value of variable $a
   $$a = 'World'; //$($a) is equals to $(hello)
   echo $hello; //$hello is World i.e. $hello is new variable with value 'World'
?>

Output:

World