The importance of using === instead of == in php!

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?

See http://stackoverflow.com/questions/1117967/what-does-mean — ChrisF, Jun 21 '10 at 12:35
@Chris. I have look at that question already. But my question is about examples, where it is really important to consider using "===" operator, not "==" operator. — Bakhtiyor, Jun 21 '10 at 12:38
@Skilldrick: No, you can't. PHP doesn't automatically convert boolean values to "true"/"false" strings. Your example would yield either `1` (for `true`) or the empty string (for `false`). The examples could be shortened, however, by using the ternary operator like so: `print ($var == false) ? 'true' : 'false';`. — Jakob, Jun 21 '10 at 13:10
One of my pet peeves in PHP, by the way. It makes quick-and-dirty debugging so much less quick and so much more dirty, when all you want is for `print true` to actually *print* true. — Jakob, Jun 23 '10 at 18:36

score 13 · Accepted Answer · edited Jun 20 '20 at 09:12

13

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.

edited Jun 20 '20 at 09:12

Community

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answered Jun 21 '10 at 12:32

Felix Kling

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1

`next` and `current` are not good examples. What if there are *false* values like in `array(false)`? – Gumbo Jun 21 '10 at 13:07
1

@Gumbo: That's right. I just mentioned them because they also have this warning box in their documentation. But there is a note (that should be more emphasized imho): *Note: You won't be able to distinguish the end of an array from a boolean FALSE element. To properly traverse an array which may contain FALSE elements, see the each() function.* – Felix Kling Jun 21 '10 at 13:15

wnrph · Answer 2 · 2010-06-21T13:03:41.747

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.

score 6 · Answer 3 · edited Jun 21 '10 at 13:59

6

In strpos() you have 0 when string is found and false when is misissing. You must use === to check difference.

edited Jun 21 '10 at 13:59

Annika Backstrom

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answered Jun 21 '10 at 12:35

user372146

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Sarfraz · Answer 4 · 2010-06-21T12:37:44.880

3

The === is strict type comparison operator, it will not only check values but also their type whereas == only checks whether or not values are same.

Consider a situation when you compare numbers or strings:

if (4 === 4) // same type and value
{
  // true
}

but

if (4 == "4") // same value but different type
{
  // true
}

and

if (4 === "4") // same value but different type
{
  // false
}

So in above cases, you have to make sensible choice whether to use == or ===

edited Jun 21 '10 at 12:37

answered Jun 21 '10 at 12:31

Sarfraz

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Yeah, I knew that. But I want to find out examples where it is important to use one operator instead of another. – Bakhtiyor Jun 21 '10 at 12:34
Look at Felix Kling's response for example. He understood what I want. – Bakhtiyor Jun 21 '10 at 12:35
@Bakhtiyor: As you can see in my example, this is also useful when comparing numbers and strings :) – Sarfraz Jun 21 '10 at 12:36
Now, I can see, but when commenting to you answer there was no examples. Thx a lot for example. – Bakhtiyor Jun 21 '10 at 12:40

score 3 · Answer 5 · answered Jun 21 '10 at 12:43

A good example where you can get into trouble is comparing 0 and a string, fx

if (0 == 'completed') {
  // evaluates as TRUE
}

A string not starting with a number becomes 0 when converted into an int. This can become a problem when comparing a status that can be 0 to a string.

score 2 · Answer 6 · answered Jun 21 '10 at 12:33

2

The === operator checks type as well as value equality.

That's why 0 === false does not return true, as they are not of the same type.

answered Jun 21 '10 at 12:33

Ian P

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score 1 · Answer 7 · answered Jun 21 '10 at 12:36

strpos($needle,$haystack) and related functions will return false if $needle doesn't exist in $haystack, and 0 if $needle is the first character of $haystack; and there's a whole host of similar functions. Using == can give you incorrect results in this case, so you should always use ===. The manual clarly identifies where this is necessary.

score 0 · Answer 8 · answered Jun 21 '10 at 12:35

I have recently ran into this problem myself when I was writing a quick SQL query parser. To cut the story short, I was comparing quoted parameters with their corner-case placeholders. Basically, the following code led me to some hard debugging times (the example is simplified of course)

$var = 0; // This is supplied dynamically

$someCondition = $var == '?';

$someCondition ended up being true all the time. Shiver

Basically any non-strict (==) <int> == <string> comparison will cause the string to be cast to an integer. Depending on the input, this might end up as 0 or the int-value of the string, if it has any, even if the whole string is not completely numeric.

score 0 · Answer 9 · answered Jun 21 '10 at 13:15

If you compare two numerical strings, they are compared as integers.1

var_dump("15" == "0xF"); // TRUE
var_dump("15" === "0xF"); // FALSE

and TRUE is indirectly equal to FALSE 2

$a = 0;
$b = 'x';
var_dump(FALSE == $a); // TRUE
var_dump($a == $b);  // TRUE
var_dump($b == TRUE);  // TRUE
var_dump(FALSE === $a); // FALSE
var_dump($a === $b);  // FALSE
var_dump($b === TRUE);  // FALSE

The importance of using === instead of == in php!

9 Answers9