How to Fix 'unserialize(): Error at offset' in PHP Serialized Strings?

How to Repair a Corrupted Serialized String with Invalid Byte Count Length

Introduction

When dealing with serialized data, it's possible to encounter an error stating "unserialize() [function.unserialize]: Error at offset." This error typically occurs due to invalid serialization data caused by incorrect byte count length.

Why the Error Occurs

During serialization, PHP calculates the length of each element in the serialized array using the strlen() function. However, if the data contains characters, such as double quotes, that require special handling (e.g., escaping), the byte count may become incorrect.

Quick Fix

To address this error, it's necessary to recalculate the length of each element in the serialized array manually:

$data = preg_replace('!s:(\d+):"(.*?)";!e', "'s:'.strlen('').':\"\";'", $data);

Recommended Approach

In addition to the quick fix, it's important to identify the cause of the incorrect serialization. Consider the following steps:

• Ensure that double quotes (") are escaped properly, e.g., use single quotes (').
• If the data contains UTF-8 characters, use array_map("utf8_encode", $data) to encode them before serialization.
• Use base64 encoding to convert the serialized data to a safe format before storing it in the database:

$toDatabase = base64_encode(serialize($data));  // Save to database
$fromDatabase = unserialize(base64_decode($data)); //Getting saved format

Detecting Errors in Serialized Data

Although the error message provides a general indication of the problem, it's often helpful to pinpoint the exact location of the corrupted data. To do this, use the findSerializeError function, which compares two serialized strings and identifies the differences between them, including incorrect byte counts:

function findSerializeError($data1) {
    echo "<pre class="brush:php;toolbar:false">";
    $data2 = preg_replace('!s:(\d+):"(.*?)";!e', "'s:'.strlen('').':\"\";'", $data1);
    // ...
}

By providing detailed information about the error, this function helps identify and resolve serialization issues.

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