Excel公式中的“，X”是什么意思？

Question

I am trying to understand the logic of this Excel formula in particular what the lines with ,"x", mean. Can someone help me with pseudo logic or extract the math formula ?

Excel formula (in part) :

IF(INDEX(TurboCheck,pLookupIndex)="T",
    IF(J4<INDEX(BaseMiles,pLookupIndex),(INDEX(BaseMiles,pLookupIndex)-J4)*-T_Under_Base,
        IF(J4>INDEX(Max_Mileage,pLookupIndex),"x",(J4-INDEX(BaseMiles,pLookupIndex))*T_Over_Base)
    ),
    IF(J4<INDEX(BaseMiles,pLookupIndex),(INDEX(BaseMiles,pLookupIndex)-J4)*-NT_Under_Base,
        IF(J4>INDEX(Max_Mileage,pLookupIndex),"x",(J4-INDEX(BaseMiles,J8))*NT_Over_Base)
    )

) 

Attempting to build it out into (PHP) logic:

J4 = $mileage
INDEX(BaseMiles,pLookupIndex) = $BaseMiles
T_Under_Base = $T_Under_Base
T_Over_Base = $T_Over_Base
NT_Under_Base = $NT_Under_Base
NT_Over_Base = $NT_Over_Base
INDEX(Max_Mileage,pLookupIndex) = $MaxMiles
INDEX(TurboCheck,pLookupIndex) = $is_turbo



IF($is_turbo) {
    if ($mileage<$BaseMiles) {
        $calc = (($BaseMiles - $mileage) * (-1 * $T_Under_Base));
        if ($mileage > $MaxMiles) {
            $calc = (($mileage - $BaseMiles)*$T_Over_Base)
        }
    }

    else if ($mileage < $BaseMiles)
        $calc = (($mileage - $BaseMiles)  * (-1 * $NT_Under_Base));

        if ($mileage > $BaseMiles) {
            $calc = (($mileage - $BaseMiles) * $NT_Over_Base);
        }
    }

}

Obviously this isn't right but wanted to show what I had so far.

UPDATE: I have this as a working method, I haven't verified it fully yet but wanted to thank everyone for the help!

/**
 * Calculate Adjustment - adapted from excel sheet formula:
 *
 *   IF(
 *      IF(INDEX(TurboCheck,pLookupIndex)="T",
 *           IF(J4<INDEX(BaseMiles,pLookupIndex),(INDEX(BaseMiles,pLookupIndex)-J4)*-T_Under_Base,
 *              IF(J4>INDEX(Max_Mileage,pLookupIndex),"x",(J4-INDEX(BaseMiles,pLookupIndex))*T_Over_Base),
 *          IF(J4<INDEX(BaseMiles,pLookupIndex),(INDEX(BaseMiles,pLookupIndex)-J4)*-NT_Under_Base,
 *          IF(J4>INDEX(Max_Mileage,pLookupIndex),"x",(J4-INDEX(BaseMiles,pLookupIndex))*NT_Over_Base))
 *          )
 *          < (-Max_RV_Adj*J3),-Max_RV_Adj*J3,
 *     IF(INDEX(TurboCheck,pLookupIndex)="T",
 *          IF(J4<INDEX(BaseMiles,pLookupIndex),(INDEX(BaseMiles,pLookupIndex)-J4)*-T_Under_Base,
 *             IF(J4>INDEX(Max_Mileage,pLookupIndex),"x",(J4-INDEX(BaseMiles,pLookupIndex))*T_Over_Base)),
 *          IF(J4<INDEX(BaseMiles,pLookupIndex),(INDEX(BaseMiles,pLookupIndex)-J4)*-NT_Under_Base,
 *          IF(J4>INDEX(Max_Mileage,pLookupIndex),"x",(J4-INDEX(BaseMiles,J8))*NT_Over_Base)))
 *   )
 *
 * @param $filters
 * @return array
 */
private function calcAdjustment($filters)
{
    $mileage = $filters['mileage'];
    $BaseMiles = $filters['base'];
    $MaxMiles = $filters['max_mi'];
    $Max_RV_Adj_Pct = (!empty($this->lease_defaults['max_rv_adj'])) ? $this->lease_defaults['max_rv_adj'] : 10; //percentage //
    $BaseRV = $filters['base_residual_value'];
    $adjust = null;


    if ($mileage < $MaxMiles) {
        if ($filters['is_turbo']) {
            $Under_Base = $filters['under_base']['turbo'];
            $Over_Base = $filters['over_base']['turbo'];
        } else {
            $Under_Base = $filters['under_base']['base'];
            $Over_Base = $filters['over_base']['base'];
        }
        if ($mileage < $BaseMiles) {
            $adjust = (($BaseMiles - $mileage) * (-1 * $Under_Base));
        } else {
            $adjust = (($mileage - $BaseMiles) * $Over_Base);
        }
        if ($adjust < (-$Max_RV_Adj_Pct * $BaseRV)) {
            $adjust = (($mileage - $adjust) * $Over_Base);    // can be negative value
        }
    }
    if ($adjust != null) {
        $ret = array("valid" => true, "adjust" => $adjust);
    } else {
        $ret = array("valid" => true, "adjust" => null);
    }
    return $ret;
}

Answer 1

if ($is_turbo == 'T') {
  if ($mileage < $BaseMiles) {
    return (($BaseMiles - $mileage) * (-1 * $T_Under_Base));
  } else {
    if ($mileage > $MaxMiles) {
      return 'x';
    } else {
      return (($mileage - $BaseMiles)*$T_Over_Base);
    }
  }
} else {
  if ($mileage < $BaseMiles) {
    return ($BaseMiles - $mileage) * (-1 * $NT_Under_Base);
  } else {
    if ($mileage > $MaxMiles) {
      return 'x';
    } else {
      return ($mileage - $BaseMiles) * $NT_Over_Base;
    }
  }
}

provided that J8 is pLookupIndex .