How does true/false work in PHP?

Because I've visited this page several times, I've decided to post an example (loose) comparison test.

Results:

""         -> false
"0"        -> false
"0.0"      -> true
"1"        -> true
"01"       -> true
"abc"      -> true
"true"     -> true
"false"    -> true
"null"     -> true
0          -> false
0.1        -> true
1          -> true
1.1        -> true
-42        -> true
"NAN"      -> true
0          -> false
NAN        -> true
null       -> false
true       -> true
false      -> false
[]         -> false
[""]       -> true
["0"]      -> true
[0]        -> true
[null]     -> true
["a"]      -> true
{}         -> true
{}         -> true
{"t":"s"}  -> true
{"c":null} -> true

Test code:

class Vegetable {}

class Fruit {
    public $t = "s";
}

class Water {
    public $c = null;
}

$cases = [
    "",
    "0",
    "0.0",
    "1",
    "01",
    "abc",
    "true",
    "false",
    "null",
    0,
    0.1,
    1,
    1.1,
    -42,
    "NAN",
    (float) "NAN",
    NAN,
    null,
    true,
    false,
    [],
    [""],
    ["0"],
    [0],
    [null],
    ["a"],
    new stdClass(),
    new Vegetable(),
    new Fruit(),
    new Water(),
];

echo "<pre>" . PHP_EOL;

foreach ($cases as $case) {
    printf("%s -> %s" . PHP_EOL, str_pad(json_encode($case), 10, " ", STR_PAD_RIGHT), json_encode( $case == true ));
}

Summary:

• When a strict (===) comparison is done, everything except true returns false.
• an empty string ("") is falsy
• a string that contains only 0 ("0") is falsy
• NAN is truthy
• an empty array ([]) is falsy
• a container (array, object, string) that contains a falsy value is truthy
  • an exception to this is 0 in "" (see the third item)

This is covered in the PHP documentation for booleans and type comparison tables.

When converting to boolean, the following values are considered FALSE:

• the boolean FALSE itself
• the integer 0 (zero)
• the float 0.0 (zero)
• the empty string, and the string '0'
• an array with zero elements
• an object with zero member variables (PHP 4 only)
• the special type NULL (including unset variables)
• SimpleXML objects created from empty tags

Every other value is considered TRUE.

Zero is false, nonzero is true.

In php you can test more explicitly using the === operator.

if (0==false) 
    echo "works"; // will echo works

if (0===false) 
    echo "works"; // will not echo anything