PHP case-insensitive in_array function

The obvious thing to do is just convert the search term to lowercase:

if (in_array(strtolower($word), $array)) { 
  ...

of course if there are uppercase letters in the array you'll need to do this first:

$search_array = array_map('strtolower', $array);

and search that. There's no point in doing strtolower on the whole array with every search.

Searching arrays however is linear. If you have a large array or you're going to do this a lot, it would be better to put the search terms in key of the array as this will be much faster access:

$search_array = array_combine(array_map('strtolower', $a), $a);

then

if ($search_array[strtolower($word)]) { 
  ...

The only issue here is that array keys must be unique so if you have a collision (eg "One" and "one") you will lose all but one.

function in_arrayi($needle, $haystack) {
    return in_array(strtolower($needle), array_map('strtolower', $haystack));
}

From Documentation

you can use preg_grep():

$a= array(
 'one',
 'two',
 'three',
 'four'
);

print_r( preg_grep( "/ONe/i" , $a ) );

function in_arrayi($needle, $haystack) {
    return in_array(strtolower($needle), array_map('strtolower', $haystack));
}

Source: php.net in_array manual page.

/**
 * in_array function variant that performs case-insensitive comparison when needle is a string.
 *
 * @param mixed $needle
 * @param array $haystack
 * @param bool $strict
 *
 * @return bool
 */
function in_arrayi($needle, array $haystack, bool $strict = false): bool
{

    if (is_string($needle)) {

        $needle = strtolower($needle);

        foreach ($haystack as $value) {

            if (is_string($value)) {
                if (strtolower($value) === $needle) {
                    return true;
                }
            }

        }

        return false;

    }

    return in_array($needle, $haystack, $strict);

}


/**
 * in_array function variant that performs case-insensitive comparison when needle is a string.
 * Multibyte version.
 *
 * @param mixed $needle
 * @param array $haystack
 * @param bool $strict
 * @param string|null $encoding
 *
 * @return bool
 */
function mb_in_arrayi($needle, array $haystack, bool $strict = false, ?string $encoding = null): bool
{

    if (null === $encoding) {
        $encoding = mb_internal_encoding();
    }

    if (is_string($needle)) {

        $needle = mb_strtolower($needle, $encoding);

        foreach ($haystack as $value) {

            if (is_string($value)) {
                if (mb_strtolower($value, $encoding) === $needle) {
                    return true;
                }
            }

        }

        return false;

    }

    return in_array($needle, $haystack, $strict);

}

$user_agent = 'yandeX';
$bots = ['Google','Yahoo','Yandex'];        
        
foreach($bots as $b){
     if( stripos( $user_agent, $b ) !== false ) return $b;
}

Why not use a regex case insensitive search - this will ignore the case in both the haystack and the needle:

$haystack = array('MacKnight', 'MacManus', 'MacManUS', 'MacMurray');
$needle='MacMANUS';
$regex='/'.$needle.'/i';
$matches  = preg_grep ($regex, $haystack);
if($m=current($matches)){ // find first case insensitive match
   echo 'match found: '.$m;
}else{
   echo 'no match found.';
} 

• in_array accepts these parameters : in_array(search,array,type)
• if the search parameter is a string and the type parameter is set to TRUE, the search is case-sensitive.
• so in order to make the search ignore the case, it would be enough to use it like this :

$a = array( 'one', 'two', 'three', 'four' );

$b = in_array( 'ONE', $a, false );