I could only guess that it would be:
$mystery = ($a < $b) && ($b < $c) && ($c < $d) && ($d < $f);
As in, for any:
A x B y C
Where 'x' and 'y' are one of ('>','>=','<','<='),
then the equivalent would be:
(A x B) && (B y C)
Which would also hold true for:
A < B >= C -> (A < B) && (B >= C)
A > B < C -> (A > B) && (B < C)