How does true/false work in PHP?

I wonder how PHP handles true/false comparison internally. I understand that true is defined as 1 and false is defined as 0. When I do if("a"){ echo "true";} it echos "true". How does PHP recognize "a" as 1 ?

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.

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)

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

The best operator for strict checking is

if($foo === true){}

That way, you're really checking if its true, and not 1 or simply just set.

PHP uses weak typing (which it calls 'type juggling'), which is a bad idea (though that's a conversation for another time). When you try to use a variable in a context that requires a boolean, it will convert whatever your variable is into a boolean, according to some mostly arbitrary rules available here: http://www.php.net/manual/en/language.types.boolean.php#language.types.boolean.casting

think of operator as unary function: is_false(type value) which returns true or false, depending on the exact implementation for specific type and value. Consider if statement to invoke such function implicitly, via syntactic sugar.

other possibility is that type has cast operator, which turns type into another type implicitly, in this case string to Boolean.

PHP does not expose such details, but C++ allows operator overloading which exposes fine details of operator implementation.