How to Compare Date

Dear all I had a Input form for user to input the Leave Record, I want to do some check before the record can insert into database! Here is the problem i had when checking the Leave From Date and Leave To Date. I want to prevent any Leave To Date which is earlier then Leave From Date. how i can do the comparsion? I had tried to use the following script but it doesn't work! <? $a=$leave_form $b=$leave_to if ($a<$b) { insert statement } ?> It doesn't work since i use the < Operator! Does anyone know how i can perform this comparsion? -- Thx a lot! Jack nedcor4@nedcor.com.hk