<form method="post" autocomplete="off">
   <textarea id="wiyetxt" name="input"></textarea>
   <input type="submit" value="Send">
   <input type="reset" value="Clear">
</form>

$input = $_POST["input"];
$keywords = ["apple" => "Found 'apple'", "banana" => "Found 'banana'",
  "carrot" => "Found 'carrot'"];
if ($input contains one of these keys) {
  alert(value which corresponds to the key);
}
else {
  alert("No keys were found")
}