How PHP treats booleans

Because PHP is loosely typed sometimes how it treats and converts variables can be confusing. However, by running a simple test script we can get a better idea of how things work.

The tests below run three tests for each variable:

Run an equals operator (==)
Run an identical operator (===)
Cast to a bool and the examine the resulting variable

Here’s the code:

<?php 

$vars['BOOL_TRUE']        = true;
$vars['BOOL_FALSE']       = false;
$vars['INT_0']            = 0;
$vars['INT_1']            = 1;
$vars['STRING_0']         = '0';
$vars['STRING_1']         = '1';
$vars['STRING']           = 'foo';
$vars['EMPTY_STRING']     = '';
$vars['POSITIVE_INT']     = 42;
$vars['NEGATIVE_INT']     = -42;
$vars['ZERO_FLOAT']       = 0.0000;
$vars['POSITIVE_FLOAT']   = 42.1;
$vars['NEGATIVE_FLOAT']   = -42.1;
$vars['NULL']             = null;

echo '<pre>';

foreach($vars as $key => $value) {

    if($value == true) {

        echo "$key: " . 'if($value) returns true<br />';

    }
    else {

        echo "$key: " . 'if($value) returns false<br />';

    }

    if($value === true) {

        echo "$key: " . 'if($value === true) returns true<br />';

    }
    else {

        echo "$key: " . 'if($value === true) returns false<br />';

    }

    $cast = (bool) $value;

    var_dump($cast);

    echo '</pre>';
}

?>

The output looks like this:

BOOL_TRUE: if($value) returns true BOOL_TRUE: if($value === true) returns true bool(true) BOOL_FALSE: if($value) returns false BOOL_FALSE: if($value === true) returns false bool(false) INT_0: if($value) returns false INT_0: if($value === true) returns false bool(false) INT_1: if($value) returns true INT_1: if($value === true) returns false bool(true) STRING_0: if($value) returns false STRING_0: if($value === true) returns false bool(false) STRING_1: if($value) returns true STRING_1: if($value === true) returns false bool(true) STRING: if($value) returns true STRING: if($value === true) returns false bool(true) EMPTY_STRING: if($value) returns false EMPTY_STRING: if($value === true) returns false bool(false) POSITIVE_INT: if($value) returns true POSITIVE_INT: if($value === true) returns false bool(true) NEGATIVE_INT: if($value) returns true NEGATIVE_INT: if($value === true) returns false bool(true) ZERO_FLOAT: if($value) returns false ZERO_FLOAT: if($value === true) returns false bool(false) POSITIVE_FLOAT: if($value) returns true POSITIVE_FLOAT: if($value === true) returns false bool(true) NEGATIVE_FLOAT: if($value) returns true NEGATIVE_FLOAT: if($value === true) returns false bool(true) NULL: if($value) returns false NULL: if($value === true) returns false bool(false)

Here are some things to bear in mind when using the == operator:

All non-empty strings return true

Any number other than 0 (even if zero is expressed as '0' or '0.0') return true

Other useful notes:

Only true will return true when using ===, everything else is false

null always returns false

The true and false keywords are not case-sensitive (as is null)

Casting to bool always matches what == would return

Finally, according to the PHP docs, these also return false: an array with zero elements, an object with zero member variables (PHP 4 only) and SimpleXML objects created from empty tags.

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Tim Bennett is a web designer and developer. He has a First Class Honours degree in Computing from Leeds Metropolitan University and currently runs his own one-man web design company, Texelate.