In PHP, variable values have an associated “truthiness” so even non-boolean values will equate to true or false. This allows any variable to be used in a conditional block, e.g.

if ($var == true) { /* explicit version */ }
if ($var) { /* $var == true is implicit */ }

Here are some fundamental rules for different types of variable values:

• Strings with non-zero length equate to true including strings containing only whitepace such as ' '.
• Empty strings '' equate to false.

$var = '';
$var_is_true = ($var == true); // false
$var_is_false = ($var == false); // true

$var = '   ';
$var_is_true = ($var == true); // true
$var_is_false = ($var == false); // false

• Integers equate to true if they are nonzero, while zero equates to false.

$var = -1;
$var_is_true = ($var == true); // true
$var = 99;
$var_is_true = ($var == true); // true
$var = 0;
$var_is_true = ($var == true); // false

• null equates to false

$var = null;
$var_is_true = ($var == true); // false
$var_is_false = ($var == false); // true

• Empty strings '' and string zero '0' equate to false.

$var = '';
$var_is_true = ($var == true); // false
$var_is_false = ($var == false); // true

$var = '0';
$var_is_true = ($var == true); // false
$var_is_false = ($var == false); // true

• Floating-point values equate to true if they are nonzero, while zero values equates to false.

* `NAN` (PHP's Not-a-Number) equates to `true`, i.e. `NAN == true` is `true`. This is because `NAN` is a _nonzero_ floating-point value.
* Zero-values include both +0 and -0 as defined by IEEE 754. PHP does not distinguish between +0 and -0 in its double-precision floating-point, i.e. `floatval('0') == floatval('-0')` is `true`. 
    * In fact, `floatval('0') === floatval('-0')`. 
    * Additionally, both `floatval('0') == false` and `floatval('-0') == false`.

$var = NAN;
$var_is_true = ($var == true); // true
$var_is_false = ($var == false); // false

$var = floatval('-0');
$var_is_true = ($var == true); // false
$var_is_false = ($var == false); // true

$var = floatval('0') == floatval('-0');
$var_is_true = ($var == true); // false
$var_is_false = ($var == false); // true

IDENTICAL OPERATOR

In the PHP Documentation for Comparison Operators, there is an Identical Operator ===. This operator can be used to check whether a variable is identical to a reference value:

$var = null;
$var_is_null = $var === null; // true
$var_is_true = $var === true; // false
$var_is_false = $var === false; // false

It has a corresponding not identical operator !==:

$var = null;
$var_is_null = $var !== null; // false
$var_is_true = $var !== true; // true
$var_is_false = $var !== false; // true