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

This question already has answers here: How do the PHP equality (== double equals) and identity (=== triple equals) comparison operators differ? (13 answers) Closed 4 years ago.

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 开发者_Python百科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?

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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? ;))

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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.

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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.

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

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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 ===

---

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.

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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.

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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.

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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.

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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