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false is a built-in boolean literal in PHP representing the negative truth value. It is a scalar data type of bool utilized fundamentally in logical operations, state evaluation, and as a standard return value indicating the failure of an internal function.

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Syntax and Case Sensitivity

The false keyword is case-insensitive. However, the PSR-12 coding standard mandates the use of lowercase for all PHP type keywords. 
$val1 = false;
$val2 = FALSE; // Valid, but violates PSR-12
$val3 = FaLsE; // Valid, but highly discouraged

var_dump($val1 === $val2); // bool(true)

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Type Coercion and “Falsy” Values

Due to PHP’s dynamic typing system, various values of different data types are implicitly coerced to false when evaluated in a boolean context (such as a conditional expression or when explicitly cast using (bool)). These are technically referred to as “falsy” values. The following values strictly evaluate to false: 
var_dump((bool) 0);           // Integer zero
var_dump((bool) 0.0);         // Float zero (and -0.0)
var_dump((bool) "");          // Empty string
var_dump((bool) "0");         // String containing exactly "0"
var_dump((bool) []);          // Empty array
var_dump((bool) null);        // Uninitialized or null variables
Note: Unlike some other languages, an empty object (new stdClass()) evaluates to true in PHP.

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Casting false to Other Types

When false is explicitly or implicitly cast to other scalar or compound types, PHP applies specific conversion rules: 
var_dump((string) false); // string(0) "" (Evaluates to an empty string, not "0" or "false")
var_dump((int) false);    // int(0)
var_dump((float) false);  // float(0)
var_dump((array) false);  // array(1) { [0]=> bool(false) }

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Comparison Mechanics

PHP provides two comparison operators that handle false differently based on type juggling rules. Loose Comparison (==) Loose comparison applies type coercion before comparing values. false will loosely match any “falsy” value. 
var_dump(false == 0);    // bool(true)
var_dump(false == "");   // bool(true)
var_dump(false == "0");  // bool(true)
var_dump(false == []);   // bool(true)
var_dump(false == null); // bool(true)
Strict Comparison (===) Strict comparison evaluates both the value and the data type. It will not perform type coercion. false will only strictly match another boolean false. 
var_dump(false === 0);    // bool(false)
var_dump(false === "");   // bool(false)
var_dump(false === null); // bool(false)

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false as a Type Hint (PHP 8.0+)

Historically, false was only a value, not a valid type declaration.
  • PHP 8.0 introduced false as a valid literal type exclusively within Union Types (e.g., string|false), reflecting the legacy behavior of internal PHP functions that return a specific type on success and false on failure.
  • PHP 8.2 elevated false to a standalone type, allowing it to be used as a return type, parameter type, or property type declaration on its own.
// PHP 8.0+ Union Type
function findItem(): string|false {
    return false;
}

// PHP 8.2+ Standalone Type
function alwaysFails(): false {
    return false;
}
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