How do the equality (== double equals) and identity (=== triple equals) comparison operators differ?

Question

Can you explain the difference between == and ===, giving some useful examples?

Answer 1

== compares the values of variables for equality, type casting as necessary. === checks if the two variables are of the same type AND have the same value.

A full explanation of the differences are available in the PHP manual.

Here's a table I put together showing how some variables compare to each other.

// "===" means that they are identical  
// "==" means that they are equal  
// "!=" means that they aren't equal.

         false    null     array()  0        "0"      0x0      "0x0"    "000"    "0000"
false    ===      ==       ==       ==       ==       ==       !=       !=       !=    
null     ==       ===      ==       ==       !=       ==       !=       !=       !=    
array()  ==       ==       ===      !=       !=       !=       !=       !=       !=    
0        ==       ==       !=       ===      ==       ===      ==       ==       ==    
"0"      ==       !=       !=       ==       ===      ==       ==       ==       ==    
0x0      ==       ==       !=       ===      ==       ===      ==       ==       ==    
"0x0"    !=       !=       !=       ==       ==       ==       ===      ==       ==    
"000"    !=       !=       !=       ==       ==       ==       ==       ===      ==    
"0000"   !=       !=       !=       ==       ==       ==       ==       ==       ===

Answer 2

You would use === to test whether a function or variable is false rather than just equating to false (zero or an empty string).

$needle = 'a';
$haystack = 'abc';
$pos = strpos($haystack, $needle);
if ($pos === false) {
    echo $needle . ' was not found in ' . $haystack;
} else {
    echo $needle . ' was found in ' . $haystack . ' at location ' . $pos;
}

In this case strpos would return 0 which would equate to false in the test

if ($pos == false)

or

if (!$pos)

which is not what you want here.

Answer 3

In regards to Javascript

The === operator works the same as the == operator but requires that its operands have not only the same value, but also the same data type.

For example the sample below will display 'x and y are equal' but not 'x and y are identical'

var x = 4;
var y = '4';
if (x == y) {
    alert('x and y are equal');
}
if (x === y) {
    alert('x and y are identical');
}

Answer 4

Variables have a type and a value.

• $var = "test" is a string that contain "test"
• $var2 = 24 is an integer vhose value is 24.

When you use these variables (in PHP), sometimes you don't have the good type. For example, if you do

if ($var == 1) {... do something ...}

PHP have to convert ("to cast") $var to integer. In this case, "$var == 1" is true because any non-empty string is casted to 1.

When using ===, you check that the value AND THE TYPE are equal, so "$var === 1" is false.

This is useful, for example, when you have a function that can return false (on error) and 0 (result) :

if(myFunction() == false) { ... error on myFunction ... }

This code is wrong as if myFunction() returns 0, it is casted to false and you seem to have an error. The correct code is :

if(myFunction() === false) { ... error on myFunction ... }

because the test is that the return value "is a boolean and is false" and not "can be casted to false".

Answer 5

Given x = 5

1) Operator : == is "equal to". x == 8 is false
2) Operator : === is "exactly equal to" (value and type) x === 5 is true, x === "5" is false