使用多维数组调用array_diff_assoc()时出现"Array to string conversion error"错误。

以下代码行会导致数组转换成字符串错误：

$diff = array_diff_assoc($stockist, $arr);

这里，$arr 是从 JSON 文件解码得到的数组。使用 is_array() 函数，我能够验证两个参数都是数组。有人能指出问题吗？

$stockist = array();
while (!feof($file_handle)) {

    $line_of_text = fgetcsv($file_handle);
    $query = "SELECT * FROM reorderchart WHERE medicine = '"
        . trim($line_of_text[3])
        . "' ORDER BY medicine";
    $result = mysql_query($query);

    if (trim($line_of_text[2]) - trim($line_of_text[1]) <= 0) {
        
        while ($row = mysql_fetch_array($result)) {

            $file = "results.json";
            $arr = json_decode(file_get_contents($file),true);
            $pharmacy = trim($row['Medicine']);

            if (isset($stockist[$pharmacy])) {
                    
                $medicine = $stockist[$pharmacy];
                $medicine[] = trim($row['Stockist']);
                $stockist[$pharmacy] = $medicine;

            } else {

                $medicine = array();
                $medicine[] = trim($row['Stockist']);
                $stockist[$pharmacy] = $medicine;
            }
        }
    }
}
$diff = array();
$diff = array_diff_assoc($stockist,$arr);
ksort($diff);
foreach ($diff as $key => $value) {

    echo "<table align='center' border='1'>";
    echo "<tr><td align = 'center'> <font color = 'blue'> $key</td></tr>";
    
    foreach($value as $key1 => $value1) {

        echo "<tr><td align ='center'>$value1</td></tr><br>";
    }
    echo "</table>";
}