in_array

Loose checking returns some crazy, counter-intuitive results when used with certain arrays. It is completely correct behaviour, due to PHP's leniency on variable types, but in "real-life" is almost useless.
The solution is to use the strict checking option.
<?php
// Example array
$array = array(
 'egg' => true,
 'cheese' => false,
 'hair' => 765,
 'goblins' => null,
 'ogres' => 'no ogres allowed in this array'
);
// Loose checking -- return values are in comments
// First three make sense, last four do not
in_array(null, $array); // true
in_array(false, $array); // true
in_array(765, $array); // true
in_array(763, $array); // true
in_array('egg', $array); // true
in_array('hhh', $array); // true
in_array(array(), $array); // true
// Strict checking
in_array(null, $array, true); // true
in_array(false, $array, true); // true
in_array(765, $array, true); // true
in_array(763, $array, true); // false
in_array('egg', $array, true); // false
in_array('hhh', $array, true); // false
in_array(array(), $array, true); // false
?>

Here is a recursive in_array function: 
<?php
$myNumbers = [
 [1,2,3,4,5],
 [6,7,8,9,10],
];
$array = [
 'numbers' => $myNumbers
];
// Let's try to find number 7 within $array
$hasNumber = in_array(7, $array, true); // bool(false)
$hasNumber = in_array_recursive(7, $array, true); // bool(true)
function in_array_recursive(mixed $needle, array $haystack, bool $strict): bool
{
 foreach ($haystack as $element) {
 if ($element === $needle) {
 return true;
 }
 $isFound = false;
 if (is_array($element)) {
 $isFound = in_array_recursive($needle, $element, $strict);
 }
 
 if ($isFound === true) {
 return true;
 }
 }
 return false;
}

I'm not sure why PHP doesn't provide a way to specify a binary search. Here's an example of the performance gains, for this array size, about 50x improvement using interpreted PHP. If built in, it could probably achieve around 1000x improvement, again for this array size.
<?php
// Set up sorted array
$X = array(1);
for ($j = 1; $j < 50000; ++$j)
 $X[] = $X[$j - 1] + rand(1, 6);
// Using in_array
$x = -microtime(true);
$m = 0;
for ($j = 0; $j < 10000; ++$j)
 $m += in_array(rand(1, 175000), $X);
$x += microtime(true);
echo $x.PHP_EOL;
// Using binarySearch
$x = -microtime(true);
$m = 0;
for ($j = 0; $j < 10000; ++$j)
 $m += binarySearch($X, rand(1, 175000));
$x += microtime(true);
echo $x.PHP_EOL;
function binarySearch($array, $value) {
 $low = 0;
 $high = count($array) - 1;
 while ($low <= $high) {
 $pivot = floor(($low + $high) / 2);
 if ($array[$pivot] == $value)
 return true;
 if ($value < $array[$pivot])
 $high = $pivot - 1;
 else
 $low = $pivot + 1;
 }
 // No match
 return false;
}
/* Sample outputs, first is in_array, second is binarySearch
1.3544600009918
0.026464939117432
1.6158990859985
0.033976078033447
1.5184400081635
0.026461124420166
*/

I found out that in_array will *not* find an associative array within a haystack of associative arrays in strict mode if the keys were not generated in the *same order*:
<?php
$needle = array(
 'fruit'=>'banana', 'vegetable'=>'carrot'
 );
$haystack = array(
 array('vegetable'=>'carrot', 'fruit'=>'banana'),
 array('fruit'=>'apple', 'vegetable'=>'celery')
 );
echo in_array($needle, $haystack, true) ? 'true' : 'false';
// Output is 'false'
echo in_array($needle, $haystack) ? 'true' : 'false';
// Output is 'true'
?>

I had wrongly assumed the order of the items in an associative array were irrelevant, regardless of whether 'strict' is TRUE or FALSE: The order is irrelevant *only* if not in strict mode.

I got an unexpected behavior working with in_array. I'm using following code:
<?php
// ...
$someId = getSomeId(); // it gets generated/fetched by another service, so I don't know what value it will have. P.S.: it's an integer
// The actual data in my edge-case scenario:
// $someId = 0;
// $anyArray = ['dataOne', 'dataTwo'];
if (in_array($someId, $anyArray)) {
 // do some work
}
// ...
?>

With PHP7.4, in_array returns boolean true.
With PHP8.1, in_array returns boolean false.
It took me quite some time to find out what's going on.

It's worth the notice that in_array is case-sensitive.
Apart from the very first option of make all lowercase or uppercase, and then compare, I found this elegant solution:
<?php
function in_arrayi(float|int|string $needle, array $haystack):bool {
 return (bool)array_uintersect([$needle],$haystack,strcasecmp(...));
}
$needle = "abc";
$haystack = ["Abc","deF","gHi"];
$result = in_arrayi($needle,$haystack);
var_dump($result);
?>