You can use the strpos() function which is used to find the occurrence of one string inside another one:
$a = 'How are you?';
if (strpos($a, 'are') !== false) {
echo 'true';
}
Note that the use of !== false is deliberate (neither != false nor === true will return the desired result); strpos() returns either the offset at which the needle string begins in the haystack string, or the boolean false if the needle isn't found. Since 0 is a valid offset and 0 is "falsey", we can't use simpler constructs like !strpos($a, 'are').