$input = array();
$eq = $_POST['eq'];
foreach($_POST as $variable_name=>$value) {
    if ($variable_name != "eq") $input[$variable_name] = $value;
}

// E.g. above would provide:
//$input["a"] = 1.2;
//$input["b"] = 2.4;
//$input["c"] = 2.3;
//$eq = "(a*b)-2(a-c)";

foreach($input as $var=>$val) {
    $eq = str_replace($var, $val, $eq);
}

echo $eq;