check if word exist in string

$a = 'How are you?';

if (strpos($a, 'are') !== false) {
    echo 'true';
}$a = 'Hello world?';

if (strpos($a, 'Hello') !== false) { //PAY ATTENTION TO !==, not !=
    echo 'true';
}
if (stripos($a, 'HELLO') !== false) { //Case insensitive
    echo 'true';
}