兼容php二维数组取值函数array_column使用

但是低版本PHP要使用，得自己实现：

if(!function_exists('array_column')){ 
    function array_column($input, $columnKey, $indexKey=null){ 
        $columnKeyIsNumber      = (is_numeric($columnKey)) ? true : false; 
        $indexKeyIsNull         = (is_null($indexKey)) ? true : false; 
        $indexKeyIsNumber       = (is_numeric($indexKey)) ? true : false; 
        $result                 = array(); 
        foreach((array)$input as $key=>$row){ 
            if($columnKeyIsNumber){ 
                $tmp            = array_slice($row, $columnKey, 1); 
                $tmp            = (is_array($tmp) && !empty($tmp)) ? current($tmp) : null; 
            }else{ 
                $tmp            = isset($row[$columnKey]) ? $row[$columnKey] : null; 
            } 
            if(!$indexKeyIsNull){ 
                if($indexKeyIsNumber){ 
                    $key        = array_slice($row, $indexKey, 1); 
                    $key        = (is_array($key) && !empty($key)) ? current($key) : null; 
                    $key        = is_null($key) ? 0 : $key; 
                }else{ 
                    $key        = isset($row[$indexKey]) ? $row[$indexKey] : 0; 
                } 
            } 
            $result[$key]       = $tmp; 
        } 
        return $result; 
    } 
} 

$records = array( 
    array( 
        'id' => 2135, 
        'first_name' => 'John', 
        'last_name' => 'Doe'
    ), 
    array( 
        'id' => 3245, 
        'first_name' => 'Sally', 
        'last_name' => 'Smith'
    ), 
    array( 
        'id' => 5342, 
        'first_name' => 'Jane', 
        'last_name' => 'Jones'
    ), 
    array( 
        'id' => 5623, 
        'first_name' => 'Peter', 
        'last_name' => 'Doe'
    ) 
); 
$firstNames = array_column($records, 'first_name'); 
print_r($firstNames);