php variables comparison == vs ===

Question

this is code:

$s = 0;
$d = "dd";

if ($s == $d) {
    var_dump($s);
    die(var_dump($d));
}

result is:

int 0

string 'dd' (length=2)

Please explain why.

why ($s == $d) results as true?

Of course, if === is used it will results as false but why this situation requires ===? Shouldn't it be returned false in both situations?

Not sure why there are downvotes on this question. It might be simple if you know the answer, but is shows someone who is trying to understand their code and getting to the nitty-gritty of how the language works. +1 from me. — Fluffeh
– Fluffeh, Commented Sep 14, 2012 at 9:15

Mihai Iorga · Accepted Answer · 2012-09-14 09:06:50Z

4

Because (int)$d equals with 0 and 0=0

you must use strict comparison === for different character tyes (string) with (int)

Your $d is automatically converted to (int) to have something to compare.

answered Sep 14, 2012 at 9:06

Mihai Iorga

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3 Comments

Fluffeh Over a year ago

You really have your finger on the pulse tonight :) +1

Mihai Iorga Over a year ago

Tonight, it's freakin' 12PM here:)

Fluffeh Over a year ago

You should move to a place that is further up the timezone, it's after seven PM Friday night here. (Awww, that just made me realize that I am a really sad bugger - spending Friday night answering programming questions)

Fluffeh · Accepted Answer · 2012-09-14 09:07:47Z

4

When you compare a number to a string, the string is first type juggled into a number. In this case, dd ends up being juggled into 0 which means that it equates to true (0==0).

When you change the code to:

 <?php 
 $s = 1;
 $d = "dd";
 if ($s == $d)
 {
    var_dump($s);
    die(var_dump($d));
 }
 ?>

You will find that it doesn't pass the if statement at all.

You can more details by reading up on comparison operators and type juggling.

answered Sep 14, 2012 at 9:07

Fluffeh

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1 Comment

Fluffeh Over a year ago

@MihaiIorga Yeah, but I made a grand entrance! swings cape !

Cosmin · Accepted Answer · 2012-09-14 09:07:12Z

2

The string "dd" is converted to int, and thus 0.

Another example :

if ( "3kids" == 3 )
{
      return true;
}

And yes, this returns true because "3kids" is converted to 3.

=== does NOT auto convert the items to the same type.

Also : 0 == false is correct, but 0 === false is not.

See : http://php.net/manual/en/language.types.type-juggling.php

answered Sep 14, 2012 at 9:07

Cosmin

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Comments

xdazz · Accepted Answer · 2012-09-14 09:07:30Z

2

The string will try to parsed into a number, returns 0 if it is not in right number format.

answered Sep 14, 2012 at 9:07

xdazz

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Comments

Michael Laffargue · Accepted Answer · 2012-09-14 09:07:48Z

2

As seen in the php website : http://php.net/manual/en/language.operators.comparison.php

var_dump(0 == "a"); // 0 == 0 -> true

answered Sep 14, 2012 at 9:07

Michael Laffargue

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Comments

Madara's Ghost · Accepted Answer · 2012-09-14 09:07:54Z

2

In PHP, == should be pronounce "Probably Equals".

When comparing with ==, PHP will juggle the file-types to try and find a match.

A string with no numbers in it, is evaluated to 0 when evaluated as an int.

Therefore they're equals.

answered Sep 14, 2012 at 9:07

Madara's Ghost

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