在WordPress[duplicate]中调用PHP函数中的自定义字段

实现下面第二个功能的正确方法是什么?第一个函数可以正常工作,但第二个函数不能正确调用WordPress中键为的自定义字段 _ni_cost_goods .

分析错误:语法错误,意外的“=”,应为变量

<?php
function round_price( $price = null, $multiplier = 1, $nearest = 1, $minus = 0 ) {
    if ( !empty( $price ) ) {
        // strip any extra characters from price
        $price = preg_replace("/[^0-9,.]/", "", $price);
        // perform calculations
        return ( round ( ( $price * $multiplier ) / $nearest ) * $nearest ) - $minus; 
    }
}

function add_gst( _ni_cost_goods = null, $multiplier = 1.1) {
    if ( !empty( _ni_cost_goods ) ) {
        // strip any extra characters from price
        _ni_cost_goods = preg_replace("/[^0-9,.]/", "", _ni_cost_goods);
        // perform calculations
        return _ni_cost_goods * $multiplier; 
    }
}
?>