PHP why is 0=='all' true?

Question

I am reading the PHP documentation for boolean.

One of the comments says 0=='all' is true.

http://php.net/manual/en/language.types.boolean.php#86809

I want to know how it becomes true.

The documentation says all non-empty strings are true except '0'.

So 'all' is true and 0 is false.

false == true should be false.

But:

if(0=='all'){
    echo 'hello';
}else{
   echo 'how are you ';
}

prints 'hello'.

Type inference is used, causing the expression to compare false with false. — Travis J, Jun 14 '18 at 07:12

score 9 · Answer 1 · edited Jun 14 '18 at 07:18

9

In PHP, operators == and != do not compare the type. Therefore PHP automatically converts 'all' to an integer which is 0.

echo intval('all');

You can use === operator to check type:

if(0 === 'all'){
    echo 'hello';
}else{
   echo 'how are you ';
}

See the Loose comparisons table.

edited Jun 14 '18 at 07:18

Timurib

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answered Jun 14 '18 at 07:13

Bhumi Shah

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DonCallisto · Answer 2 · 2018-06-14T07:23:55.303

4

As you have as left operand an integer, php tries to cast the second one to integer. So as integer representation of a string is zero, then you have a true back. If you switch operators you obtain the same result.

As Bhumi says, if you need this kind of comparison, use ===.

edited Jun 14 '18 at 07:23

answered Jun 14 '18 at 07:10

DonCallisto

29,419
9
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score 1 · Answer 3 · answered Jun 14 '18 at 07:26

If you put a string as condition in a IF steatment it is checked to be not empty or '0', but if you compare it with an integer (==, <, >, ...) it is converted to 0 int value.

if('all')
    echo 'this happens!';
if('all'>0 || 'all'<0)
    echo 'this never happens!';

PHP why is 0=='all' true?

3 Answers3