Booleans, null and which values count as false

A shop’s order form sends a quantity of 0 to remove an item, and the order is saved with a quantity of 1. The line $qty = $form['qty'] ?: 1; replaced the real 0, because PHP counts '0', like 0 and an empty string, as false.

This lesson shows which values count as false in PHP, then sorts out the checks and operators that look alike, isset(), empty() and is_null(), ?? and ?:, so that a default fills in what is missing and never what is zero.

Eight values count as false, and nearly every other value counts as true

PHP has two booleans, true and false, and the two literals may be written in any case. Booleans come out of comparisons and checks, and PHP also converts any other value to one of them whenever a condition needs it: in an if, in ! and in &&. This program converts fourteen values with (bool):

<?php

$values = [false, 0, 0.0, -0.0, '', '0', [], null, '0.0', ' ', '00', 'false', [0], -1];

foreach ($values as $value) {
    $shown = is_array($value) ? json_encode($value) : var_export($value, true);
    printf("%-8s %s\n", $shown, var_export((bool) $value, true));
}

false    false
0        false
0.0      false
-0.0     false
''       false
'0'      false
[]       false
NULL     false
'0.0'    true
' '      true
'00'     true
'false'  true
[0]      true
-1       true

The first eight lines are the values that count as false: false, the integer 0, the floats 0.0 and -0.0, the empty string, the string '0', the empty array and null. The manual adds only some internal objects, such as a GMP number equal to 0. Everything else is true, and the last six lines are the surprises: '0.0', a single space, '00', the word 'false', an array that holds a 0, and -1. Step through the rule, one type at a time:

Which values count as false depends on the type and the value, never on what a piece of text says: 'false' is a non-empty string, so it is true. The value nullnull: The value of a variable that has no value: one that was never set, or was set to null. It counts as false, turns into empty text, and isset() and ?? treat it like a missing value. is the only value of its own type, the value of a variable that was never set or was set to null, and it is always false.

isset(), empty() and null coalescing ask different questions

Form data and settings arrive as arrays, in which a key may be missing, may hold null, or may hold an empty string or '0'. This program asks the same four questions about five keys:

<?php

$form = ['coupon' => null, 'note' => '', 'qty' => '0', 'size' => 'M'];

echo "key     isset empty ??    ?:\n";
foreach (['gift', 'coupon', 'note', 'qty', 'size'] as $key) {
    $isset = var_export(isset($form[$key]), true);
    $empty = var_export(empty($form[$key]), true);
    $coalesce = var_export($form[$key] ?? 'x', true);
    $ternary = var_export($form[$key] ?: 'x', true);
    printf("%-7s %-5s %-5s %-5s %s\n", $key, $isset, $empty, $coalesce, $ternary);
}

var_dump(is_null($form['coupon']), array_key_exists('coupon', $form), array_key_exists('gift', $form));

key     isset empty ??    ?:

Warning: Undefined array key "gift" in form_defaults.php on line 10
gift    false true  'x'   'x'
coupon  false true  'x'   'x'
note    true  true  ''    'x'
qty     true  true  '0'   'x'
size    true  false 'M'   'M'
bool(true)
bool(true)
bool(false)

isset() was true only for keys that exist and are not null. empty() was true for the missing key and for every value that counts as false, '0' included, which surprises form handlers. ?? is the null coalescing operatornull coalescing operator: ??, which returns its left side unless that is null or does not exist, and its right side otherwise. It gives no warning for a missing variable or key, and it keeps 0 and empty text.: it returned its left side unless that was null or missing, so qty kept its '0' and the missing gift got the default 'x', without a warning. ?:, the short ternary, fell back on every value that counts as false, so it replaced '' and '0' too, the bug of the opening, and it warned about the missing key. is_null() was true for the coupon, but only array_key_exists() told a key set to null from a key that is not there.

Three more tools build on these rules:

• ??= assigns a default only when the variable or the key is missing or null.
• The nullsafe operator ?-> stops at the first null in a chain of properties or method calls and gives null.
• filter_var() with FILTER_VALIDATE_BOOL turns text such as yes, on and 1 into true, and no, off and 0 into false.

Defaults that keep 0, and null chains that do not crash Pro: opens with a Pro, Premium or Ultimate pass

Booleans from text: filter_var(), (bool) traps and NAN Pro: opens with a Pro, Premium or Ultimate pass

Practise: defaults that keep a zero

Then answer the lesson’s questions, which start where you are weakest:

Interview questions

Key takeaways

• Eight values count as false, false, 0, 0.0, -0.0, '', '0', [] and null; nearly all others count as true, '0.0', 'false' and -1 among them.
• Booleans come from comparisons and checks, and a condition converts any other value to one of the two booleans.
• isset() is false for a missing key and for null, empty() is true for both and for every falsy value, and only array_key_exists() tells null from missing.
• ?? replaces only a missing or null value and ?: every falsy one, so a quantity of 0 needs ??.