array_search() searches an associative array for a specified value and returns its key if found, otherwise false.    mixed array_search(mixed needle, array array [, bool strict = false])

<?PHP
	$arr=array("apple"=>"carbon","rice"=>"carbon","nuts"=>"fat","meat"=>"protein");
    $key=array_search("fat",$arr);
	echo $key; //nuts 
?>

If there are more than one key with the same value, the first occurrence will be returned. If the strict parameter is true, then the value comparison will be type sensitive.

<?PHP
	$arr=array("apple"=>"carbon","rice"=>"carbon","nuts"=>"fat","meat"=>"protein");
    $key=array_search("carbon",$arr);
	echo $key; //apple
	
    $arr=array("apple"=>"carbon","rice"=>"carbon","nuts"=>"43","meat"=>"protein");
    $key=array_search(43,$arr, true);
    echo $key; //no output, 43 and "43" types not match
	
    $key=array_search("43",$arr, true);
    echo $key; //nuts
?>

If all keys with the same value need to be retrieved, you can use array_keys() function.

<?PHP
	$arr=array("apple"=>"carbon","rice"=>"carbon","nuts"=>"fat","meat"=>"protein");
    $arr2=array_keys($arr,"carbon");
	foreach($arr2 as $element) echo "$element, "; //apple, rice, 
    $arr2=array_keys($arr,"fat");
	foreach($arr2 as $element) echo "$element, "; //nuts,  
?>

in_array() checks whether a value is in a array.

<?PHP
	$arr=array(1,2,3,4,5,6,7,8,9);
    if (in_array(3,$arr)) echo "3 is an array element";
	//3 is an array element
	
    $arr=array("2.3",4,9,10);
	if (in_array("4",$arr,true)) echo "4 is in the array";
	if (in_array(2.3,$arr,true)) echo "2.3 is in the array";
	//no output, type not match. "2.3" is string, 2.3 is number
?>