PHP is_array returns "1" on true and breaks on false

PHP is_array returns "1" on true and breaks on false

I am passing a jQuery array to PHP. My call back function returns the
value when it is a string, but breaks when sending an array. The break is
in the PHP code.
I've tested the following:
$yes = array('this', 'is', 'an array');
echo is_array($yes);
Which returns "1".
$yes = "array";
echo is_array($yes);
Which breaks the code.
Why am I not returning "true" and "false"?