PHP errors on if syntax

Hmm I learned that 'Learning PHP' that the '=' operator returns the right side value for example, A=1234 returns 1234.

so I edited

if($oper1 && $oper2){

to

if($oper1 = filter_input(INPUT_POST, 'operand1', FILTER_VALIDATE_FLOAT) &&
    $oper2 = filter_input(INPUT_POST, 'operand2', FILTER_VALIDATE_FLOAT)){

Did I make a mistake???

the entire source code is below: