Búsqueda PHP MySQL con múltiples criterios

En lugar de tomar estas variables, debe usar el operador ".=".

/*      $t1 = '';
        $t2 = '';
        $t3 = '';
        $t4 = '';
        $t5 = '';
*/
        $q = "SELECT * FROM `property` WHERE `status`='1' ";

// You need to enclose all **OR** logical tests in parenthesis.
// Moreover most of the usages of isset function are useless,
// as your are initializing many variables

        if($keyword && !empty($keyword)){
            $q .= " AND (`p_title` LIKE '%".$keyword."%' OR `address` LIKE '%".$keyword."%' OR `area` LIKE '%".$keyword."%')";
        }
        if($prop_for && !empty($prop_for)){
// If you are using double quotes you really don't need  handle to concatenation.
        $q .= " AND `p_for`='$prop_for'";
        }
        if($state && !empty($state)){
            $q .= " AND `state`='$state'";
        }
        if($ptype && !empty($ptype)){
            $q .= " AND `p_category`='$ptype'";
        }
        //min only
        if($min_price && !empty($min_price)){
            $q .= " AND `price` >= '".$min_price."'";
        }
        //max only
        if($max_price && !empty($max_price)){
            $q .= " AND `price` <= '$max_price'";
        }

// When you are not using OFFSET keyword,
//the first number after LIMIT keyword should be the number of records
                $q .= " ORDER BY `posted_date` DESC LIMIT $per_page , $start;";
                $sql = $mysqli->query($q);