// Once upon a time there was a farmer

// He had multiple haystacks
$haystackOne = range(1, 10);
$haystackTwo = range(11, 20);
$haystackThree = range(21, 30);

// In one of these haystacks he lost a needle
$needle = rand(1, 30);

// He wanted to know in what haystack his needle was
// And so he programmed...
if (in_array($needle, $haystackOne)) {
    echo "The needle is in haystack one";
} elseif (in_array($needle, $haystackTwo)) {
    echo "The needle is in haystack two";
} elseif (in_array($needle, $haystackThree)) {
    echo "The needle is in haystack three";
}

// The farmer now knew where to find his needle
// And he lived happily ever after

If you need it the other way round, i.e. to find if any of array elements contain a specific string, you can do it like this

$array = ["they has mystring in it", "some", "other", "elements"];
if (stripos(json_encode($array),'mystring') !== false) {
    echo "found mystring";
}

Though there is a chance for incorrect results:

• a false negative result if your array elements contain characters that json_encode() will escape (e.g if mystring is http://google.com/)
• a false positive result, e.g if mystring is "some","other". This code will tell you "found" while none of array elements contain such a string.

Hence explicit loop will be more bulletproof (or a brand new PHP function array_find()).

Using the instruction if?

if(isset($something['say']) && $something['say'] === 'bla') {
    // do something
}

By the way, you are assigning a value with the key say twice, hence your array will result in an array with only one value.