You could use regular expressions as its better for word matching compared to
strpos, as mentioned by other users. A strpos check for are will also return
true for strings such as: fare, care, stare, etc. These unintended matches can
simply be avoided in regular expression by using word boundaries.
A simple match for are could look something like this:
$a = 'How are you?';
if (preg_match('/are/', $a)) {
echo 'true';
}