PHP访问$item["@attributes"]["tag"]触发致命错误的技术求助

Let's break down why you're hitting this error and how to fix it.

The Root Cause #

Looking at your array structure and code, the problem comes from an incorrect traversal level in your loop.

Your converted array has this hierarchy:

$xml_ray
└── hold
    └── rec
        └── datafield → array of individual datafield items (each with @attributes and subfield)

But your code is looping over $xml_ray['hold']['rec'] directly:

foreach ($xml_ray['hold']['rec'] as $key_item => $item) {
    if ($item["@attributes"]["tag"] == $field) { // Error here
        // ...
    }
}

In this loop, $item is actually the datafield array itself (not a single datafield entry). When you try to access $item["@attributes"], that key doesn't exist in the datafield array (its keys are numeric indexes like 0, 1), so PHP returns null. Trying to access ["tag"] on null triggers the "Cannot use string offset as an array" error (PHP treats null as an empty string in this context, hence the string offset error).

The Fix #

Adjust your loop to target the datafield array directly, then iterate over each individual datafield entry. Also add isset() checks to guard against missing keys:

// First, get the array of datafield items
$datafields = $xml_ray['hold']['rec']['datafield'];

foreach ($datafields as $key_item => $item) {
    // Guard against missing @attributes or tag
    if (!isset($item["@attributes"]["tag"]) || $item["@attributes"]["tag"] !== $field) {
        continue;
    }

    $info_msg = "FIELD: $field <br />";
    $outcame_msg .= "Found correct field $field<br />";

    foreach ($item['subfield'] as $key_subfield => $subfield) {
        // Guard against missing subfield attributes/values
        if (!isset($subfield['@attributes']['code'], $subfield['@value'])) {
            continue;
        }

        $code = $subfield['@attributes']['code'];
        $value = $subfield['@value'];
        $info_holding_msg .= "$code = $value<br />";

        if ($value == $li) {
            $outcame_msg .= "Found correct li in subfield $code<br />";
            $checksum++;
        }
        if ($value == $loc) {
            $outcame_msg .= "Found correct loc in subfield $code<br />";
            $checksum++;
        }
        if ($checksum == 2 && $code == $subfield_file) {
            $outcame_msg .= "Found subfield $code with value: $value<br />";
            $checksum++;
        }
        if ($checksum == 3) {
            // Update the value using the correct array path
            $xml_ray['hold']['rec']['datafield'][$key_item]['subfield'][$key_subfield]['@value'] = $new_code;
            $outcame_msg .= "data found, changing data accepted.";
        }
    }
}

Key Changes: #

1. Target the right array: We loop over $xml_ray['hold']['rec']['datafield'] instead of $xml_ray['hold']['rec'] to get individual datafield entries.
2. Add safety checks: isset() prevents errors if any expected keys are missing (common when dealing with XML-to-array conversions).
3. Correct update path: When modifying the subfield value, we use the full path to the datafield array ($xml_ray['hold']['rec']['datafield'][$key_item]) instead of the old $xml_ray['hold']['rec'][$key_item].

Debugging Tip #

If you ever hit similar array traversal issues, add a var_dump($item) inside your loop to inspect the structure of the variable you're working with. This will immediately show you if you're at the wrong level in the array hierarchy.

内容的提问来源于stack exchange，提问作者newert