An example of repairing a serialized PHP string. See: https://stackoverflow.com/a/21389439/1170664

repair-serialized-string.php

<?php

// @see https://stackoverflow.com/a/21389439/1170664

namespace Corcel\Model\Meta;

PostMeta::where('meta_key', '_wp_attachment_metadata')->chunk(100, function ($collection) {
    foreach ($collection as $meta) {
        // recalculate the length of the elements in the serialized array
        $fixed_data = preg_replace_callback('!s:(\d+):"(.*?)";!', function($match) {
            return ($match[1] == strlen($match[2])) ? $match[0] : 's:' . strlen($match[2]) . ':"' . $match[2] . '";';
        }, $meta->meta_value);
        $meta->meta_value = $fixed_data;
        $meta->save();
    }
});