# given three equal variables
if (($a == $b ) == ($b == $c)) { print "yes!!!\n"; }

# as pointed out by Sidhekin
# given three different variables
if (($a == $b ) == ($b == $c)) { print "yes!!!\n"; }

# still prints 'yes!!!\n'.  Why?
# both sides evaluate to false or 0, 
# and 0 == 0 evaluates to true