{dev_notes}

PHP Truthy, Falsey

Published 3 years ago.
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A falsey value is considered false when encountered in a Boolean context.
A truthy value is considered true when encountered in a Boolean context.

The idea of truthy and falsey values allow comparison between what would otherwise be two incomparable data types in a programming language.

For instance, in PHP you can compare a number to a letter:

if (1 > 'a') {
    echo 'truthy';
} else {
    echo 'falsey';
}

Type Juggling

Type juggling is the process of converting a value to a comparable data type. This is done before a boolean comparison is made. These rules also apply to comparisons using the switch statement.

In PHP if a number is compared with a string, or both values are numerical strings, then each string is converted to a number and the comparison is performed numerically.

How the “truthitude” of a value is determined for a given type in a given language can be pretty arbitrary.

When PHP converts a data type to a boolean for comparison, the following values are considered FALSE:

  • The boolean FALSE itself.
  • The integer 0 (zero).
  • The float 0.0 (zero).
  • An 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.

Comparison Operators

Comparison operators allow you to compare two values.

Symbol Name Description
== Equal Checks if the values are equal after type juggling.
=== Identical Does a type-dependent check. Using this operator basically nullifies truthy/falsey as the comparison also checks if the data types of the values are identical.
!= Not equal
<> Not equal
!== Not identical
< Less than
> Greater than
<= Less than or equal to
>= Greater than or equal to
<=> Spaceship An integer less than, equal to, or greater than zero when $a is less than, equal to, or greater than $b, respectively. Available as of PHP 7.

Tests

if ($test_var) {
    echo 'true';
} else {
    echo 'false';
}

# `$test_var` example definitions and results as given by the structure above:

$test_var = '';     // false
$test_var = "";     // false
$test_var = '0';    // false
$test_var = "0";    // false
$test_var = 0;      // false
$test_var = 0.0;    // false
$test_var = [];     // false
$test_var;          // false - NOTICE Undefined variable...
(not defined)       // false - NOTICE Undefined variable...

$test_var = 1;      // true
$test_var = 'a';    // true
$test_var = "a";    // true
$test_var = a;      // true - NOTICE Use of undefined constant a...

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