PHP Boolean Casting Gotchas

This post originally appeared on Chris DeLuca's blog

I find PHP’s boolean casting rules strange and hard to remember. I know I’m not alone.

To help myself out, and anyone else that happens to stumble upon it, I wrote a commented script that demonstrates how each kind value is cast to booleans.

You can download the script to run it yourself, or view it below (or both!).

You can also skip to the results of the script (aka, what booleans are cast to!)

I added the comment “Uh oh!” where I thought the result was unexpected or inconsistent.

<?php
class fooClass {
  function do_foo() {
    echo "Hello, world";
  }
}
$fooNull = NULL;
$fooFalse = FALSE;
$fooTrue = TRUE;
$fooFalseyString = "";
$fooTruthyString = "hello";
$fooFalseyArray = [];
$fooTruthyArray = ["hello", "everyone"];
$fooFalseyNumber = 0;
$fooTruthyNumber = 5;
$fooTruthyObject = new fooClass;
$fooFalseyObject = (object) array();
$fooNegativeNumber = -2;
$fooNegativeZero = -0;
echo "PHP Version: " . phpversion() . "\n";
echo "\"Uh oh!\" added after every logically unexpected result.\n\n";
// NULL {{{

if ($fooNull) {
  echo "Default if evaluates null to true // Uh oh! \n";
} else {
  echo "Default if evaluates null to false\n";
}
if (isset($fooNull)) {
  echo "isset() if evaluates null to true\n";
} else {
  echo "isset() if evaluates null to false // Uh oh! \n";
}
if (empty($fooNull)) {
  echo "empty() if evaluates null to true\n";
} else {
  echo "empty() if evaluates null to false // Uh oh! \n";
}
echo "\n";
// }}}

// Unset variable {{{

if ($fooUnset) {
  echo "Default if evaluates an unset variable to true // Uh oh! \n";
} else {
  echo "Default if evaluates unset variable to false\n";
}
if (isset($fooUnset)) {
  echo "isset() if evaluates an unset variable to true // Uh oh! \n";
} else {
  echo "isset() if evaluates unset variable to false\n";
}
if (empty($fooUnset)) {
  echo "empty() if evaluates an unset variable to true\n";
} else {
  echo "empty() if evaluates unset variable to false // Uh oh! \n";
}
echo "\n";
// }}}

// FALSE {{{

if ($fooFalse) {
  echo "Default if evaluates false to true // Uh oh! \n";
} else {
  echo "Default if evaluates false to false\n";
}
if (isset($fooFalse)) {
  echo "isset() if evaluates false to true\n";
} else {
  echo "isset() if evaluates false to false // Uh oh! \n";
}
if (empty($fooFalse)) {
  echo "empty() if evaluates false to true\n";
} else {
  echo "empty() if evaluates false to false // Uh oh! \n";
}
echo "\n";
// }}}

// TRUE {{{

if ($fooTrue) {
  echo "Default if evaluates true to true\n";
} else {
  echo "Default if evaluates true to false\n";
}
if (isset($fooTrue)) {
  echo "isset() if evaluates true to true\n";
} else {
  echo "isset() if evaluates true to false // Uh oh! \n";
}
if (empty($fooTrue)) {
  echo "empty() if evaluates true to true // Uh oh! \n";
} else {
  echo "empty() if evaluates true to false\n";
}
echo "\n";
// }}}

// Falsey string {{{

if ($fooFalseyString) {
  echo "Default if evaluates a falsey string to true // Uh oh! \n";
} else {
  echo "Default if evaluates a falsey string to false\n";
}
if (isset($fooFalseyString)) {
  echo "isset() if evaluates a falsey string to true // Uh oh! \n";
} else {
  echo "isset() if evaluates a falsey string to false\n";
}
if (empty($fooFalseyString)) {
  echo "empty() if evaluates a falsey string to true\n";
} else {
  echo "empty() if evaluates a falsey string to false // Uh oh! \n";
}
echo "\n";
// }}}

// Truthy string {{{

if ($fooTruthyString) {
  echo "Default if evaluates a truthy string to true\n";
} else {
  echo "Default if evaluates a truthy string to false // Uh oh! \n";
}
if (isset($fooTruthyString)) {
  echo "isset() if evaluates a truthy string to true\n";
} else {
  echo "isset() if evaluates a truthy string to false // Uh oh! \n";
}
if (empty($fooTruthyString)) {
  echo "empty() if evaluates a truthy string to true // Uh oh! \n";
} else {
  echo "empty() if evaluates a truthy string to false\n";
}
echo "\n";
// }}}

// Falsey array {{{

if ($fooFalseyArray) {
  echo "Default if evaluates a falsey array to true // Uh oh! \n";
} else {
  echo "Default if evaluates a falsey array to false\n";
}
if (isset($fooFalseyArray)) {
  echo "isset() if evaluates a falsey array to true // Uh oh! \n";
} else {
  echo "isset() if evaluates a falsey array to false\n";
}
if (empty($fooFalseyArray)) {
  echo "empty() if evaluates a falsey array to true\n";
} else {
  echo "empty() if evaluates a falsey array to false // Uh oh! \n";
}
echo "\n";
// }}}

// Truthy array {{{

if ($fooTruthyArray) {
  echo "Default if evaluates a Truthy array to true\n";
} else {
  echo "Default if evaluates a Truthy array to false // Uh oh! \n";
}
if (isset($fooTruthyArray)) {
  echo "isset() if evaluates a Truthy array to true\n";
} else {
  echo "isset() if evaluates a Truthy array to false // Uh oh! \n";
}
if (empty($fooTruthyArray)) {
  echo "empty() if evaluates a Truthy array to true // Uh oh! \n";
} else {
  echo "empty() if evaluates a Truthy array to false\n";
}
echo "\n";
// }}}

// Falsey number {{{

if ($fooFalseyNumber) {
  echo "Default if evaluates a Falsey number to true // Uh oh! \n";
} else {
  echo "Default if evaluates a Falsey number to false\n";
}
if (isset($fooFalseyNumber)) {
  echo "isset() if evaluates a Falsey number to true // Uh oh! \n";
} else {
  echo "isset() if evaluates a Falsey number to false\n";
}
if (empty($fooFalseyNumber)) {
  echo "empty() if evaluates a Falsey number to true\n";
} else {
  echo "empty() if evaluates a Falsey number to false // Uh oh! \n";
}
echo "\n";
// }}}

// Truthy number {{{

if ($fooTruthyNumber) {
  echo "Default if evaluates a Truthy number to true\n";
} else {
  echo "Default if evaluates a Truthy number to false // Uh oh! \n";
}
if (isset($fooTruthyNumber)) {
  echo "isset() if evaluates a Truthy number to true\n";
} else {
  echo "isset() if evaluates a Truthy number to false // Uh oh! \n";
}
if (empty($fooTruthyNumber)) {
  echo "empty() if evaluates a Truthy number to true // Uh oh! \n";
} else {
  echo "empty() if evaluates a Truthy number to false\n";
}
echo "\n";
// }}}

// Truthy object {{{

if ($fooTruthyObject) {
  echo "Default if evaluates a Truthy object to true\n";
} else {
  echo "Default if evaluates a Truthy object to false // Uh oh! \n";
}
if (isset($fooTruthyObject)) {
  echo "isset() if evaluates a Truthy object to true\n";
} else {
  echo "isset() if evaluates a Truthy object to false // Uh oh! \n";
}
if (empty($fooTruthyObject)) {
  echo "empty() if evaluates a Truthy object to true // Uh oh! \n";
} else {
  echo "empty() if evaluates a Truthy object to false\n";
}
echo "\n";
// }}}

// Falsey object {{{

if ($fooFalseyObject) {
  echo "Default if evaluates a Falsey object to true // Uh oh! \n";
} else {
  echo "Default if evaluates a Falsey object to false\n";
}
if (isset($fooFalseyObject)) {
  echo "isset() if evaluates a Falsey object to true // Uh oh! \n";
} else {
  echo "isset() if evaluates a Falsey object to false\n";
}
if (empty($fooFalseyObject)) {
  echo "empty() if evaluates a Falsey object to true\n";
} else {
  echo "empty() if evaluates a Falsey object to false // Uh oh! \n";
}
echo "\n";
// }}}

// Negative number {{{

if ($fooNegativeNumber) {
  echo "Default if evaluates a Negative number to true\n";
} else {
  echo "Default if evaluates a Negative number to false // Uh oh! \n";
}
if (isset($fooNegativeNumber)) {
  echo "isset() if evaluates a Negative number to true\n";
} else {
  echo "isset() if evaluates a Negative number to false // Uh oh! \n";
}
if (empty($fooNegativeNumber)) {
  echo "empty() if evaluates a Negative number to true // Uh oh! \n";
} else {
  echo "empty() if evaluates a Negative number to false\n";
}
echo "\n";
// }}}

// Negative Zero {{{

if ($fooNegativeZero) {
  echo "Default if evaluates a Negative Zero to true // Uh oh! \n";
} else {
  echo "Default if evaluates a Negative Zero to false\n";
}
if (isset($fooNegativeZero)) {
  echo "isset() if evaluates a Negative Zero to true // Uh oh! \n";
} else {
  echo "isset() if evaluates a Negative Zero to false\n";
}
if (empty($fooNegativeZero)) {
  echo "empty() if evaluates a Negative Zero to true\n";
} else {
  echo "empty() if evaluates a Negative Zero to false // Uh oh! \n";
}
echo "\n";
// }}}

/* vim: set foldmethod=marker */

Here’s what the above script evaluates to (on my machine).

PHP Version: 7.3.11
"Uh oh!" added after every logically unexpected result.
Default if evaluates null to false
isset() if evaluates null to false // Uh oh!
empty() if evaluates null to true
Default if evaluates unset variable to false
isset() if evaluates unset variable to false
empty() if evaluates an unset variable to true
Default if evaluates false to false
isset() if evaluates false to true
empty() if evaluates false to true
Default if evaluates true to true
isset() if evaluates true to true
empty() if evaluates true to false
Default if evaluates a falsey string to false
isset() if evaluates a falsey string to true // Uh oh!
empty() if evaluates a falsey string to true
Default if evaluates a truthy string to true
isset() if evaluates a truthy string to true
empty() if evaluates a truthy string to false
Default if evaluates a falsey array to false
isset() if evaluates a falsey array to true // Uh oh!
empty() if evaluates a falsey array to true
Default if evaluates a Truthy array to true
isset() if evaluates a Truthy array to true
empty() if evaluates a Truthy array to false
Default if evaluates a Falsey number to false
isset() if evaluates a Falsey number to true // Uh oh!
empty() if evaluates a Falsey number to true
Default if evaluates a Truthy number to true
isset() if evaluates a Truthy number to true
empty() if evaluates a Truthy number to false
Default if evaluates a Truthy object to true
isset() if evaluates a Truthy object to true
empty() if evaluates a Truthy object to false
Default if evaluates a Falsey object to true // Uh oh!
isset() if evaluates a Falsey object to true // Uh oh!
empty() if evaluates a Falsey object to false // Uh oh!
Default if evaluates a Negative number to true
isset() if evaluates a Negative number to true
empty() if evaluates a Negative number to false
Default if evaluates a Negative Zero to false
isset() if evaluates a Negative Zero to true // Uh oh!
empty() if evaluates a Negative Zero to true