Equal (==), identical (===) and array comparison in PHP

Equal (==)

If you use equal (==), you are allowing type conversion which means PHP will try to convert the two sides into the same type and then do the comparison. So even if the two sides are NOT the same thing, they MAY still be treat as the SAME.

Consider this code:

<?php 
$left = "C"; 
$right = 0; 
var_dump($left == $right); 
?> 

Output:

bool(true)

"C" equals to 0 ?? The logic behind is : $left is a String of "C", since it is compared to $right which is a number, PHP will first convert the String "C" to a number by parsing "C" as a numeric value which is unfortunately 0, then this 0 is compares to $right which is 0, so although strange the comparison result is logically "true".

Identical (===)

On the contrary, when identical (===) is used in the comparison, PHP will not do any type conversion. PHP firstly check if the both side is of the same type. If not, then just return false. If they are of the same type, it then compare the values to see if they are the same. So it should be no wonder that the output of the below codes is "false":

<?php 
$left = "5"; 
$right = 5; 
var_dump($left === $right); 
?> 

Output:

bool(false)

What if they are Arrays?

Consider this code:

<?php 
$a = array('a'=>1, 'b'=>2, 'c'=>3);                 //reference array 
$b = array('a'=>1, 'b'=>2, 'c'=>3);                //equal and identical 
$c = array('a'=>1, 'b'=>2);                                //one element less 
$d = array('a'=>1, 'b'=>100, 'c'=>3);          //one element has different value 
$e = array('a'=>1, 'c'=>3, 'b'=>2);               //same key-value pairs but different sequence 
echo '$a == $b is ', var_dump($a ==$b); 
echo '$a === $b is ', var_dump($a === $b); 
echo '$a == $c is ', var_dump($a ==$c); 
echo '$a === $c is ', var_dump($a === $c); 
echo '$a == $d is ', var_dump($a ==$d); 
echo '$a === $d is ', var_dump($a === $d); 
echo '$a == $e is', var_dump($a ==$e); 
echo '$a === $e is', var_dump($a === $e); 
?> 

Output:

$a == $b is bool(true) 
$a === $b is bool(true) 
$a == $c is bool(false) 
$a === $c is bool(false) 
$a == $d is bool(false) 
$a === $d is bool(false) 
$a == $e is bool(true) 
$a === $e is bool(false) 

So we conclude that:

  • When two arrays are same in each key/value pair, and they have the same amount of elements, and the elements are in the same sequence, they are equal (==) and identical (===),
  • If one array has less elements than another one, they are neither equal (==) nor identical (===).
  • If one of the elements in an array has different value, the two arrays are neither equal (==) nor identical (===)
  • If two arrays have the same element, but different sequence, they are equal (==) but NOT identical (===).

Reference:

  1. Type conversion during comparison in PHP (they call it type juggling): http://php.net/manual/en/language.types.type-juggling.php
  2. Type comparisons in PHP: http://php.net/manual/en/types.comparisons.php

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