The importance of using === instead of == in php! [duplicate]

Question

Today only I have noticed and found out the importance of using === operator. You can see it in the following example:

$var=0;
if ($var==false) echo "true"; else echo "false";  //prints true
$var=false;
if ($var==false) echo "true"; else echo "false";  //prints true
$var=0;
if ($var===false) echo "true"; else echo "false";  //prints false
$var=false;
if ($var===false) echo "true"; else echo "false";  //prints true 

The question is that, are there any situations where it is important to use === operator instead of using == operator?

Answer 1

Of course, just one example: array_search()

Warning

This function may return Boolean FALSE, but may also return a non-Boolean value which evaluates to FALSE, such as 0 or "". Please read the section on Booleans for more information. Use the === operator for testing the return value of this function.

Basically if you use any function that returns a value on success but FALSE on failure, you should check the result with === to be sure (otherwise why would there be a big red warning box? ;))

---

Further examples: next(), current()
or as also mentioned string functions as strpos(), stripos(), etc.

Even substr() although it is not mentioned explicitly:

Returns the extracted part of string or FALSE on failure.

But what if the extracted part is"0"? It also evaluates to FALSE, but it is not an error.

Answer 2

Always choose === over == except you're absolutely sure you need ==, because == is not transitive. And that in turn is important for your reasoning about your code.

Consider the following code snippet

if ( $a == $b && $b == $c ) {
    // [1] assert: $a == $c
}

Anybody would infer from the if condition that the assertion $a == $c is true because we are so used to the equality relation being transitive. But that doesn't hold for ==, counter example:

$a = "0";
$b = 0;
$c = null;

Now think about how often we make (at times unconsciously) that assumption while writing code. That could lead to serious bugs.