How to Use PHP’s pack() Function to Encode GPS Coordinates and Timestamps for IoT Device Transmission

M66 2025-06-26

1. Why use binary packing?

GPS coordinates are usually represented as floating-point numbers, for example:

Latitude: 31.2304
Longitude: 121.4737
Timestamp: for instance, a Unix timestamp 1716720000

If sent directly as a string format, such as JSON, the size is relatively large and not suitable for bandwidth-constrained IoT devices. Using the pack() function, we can encode these values into a fixed-length binary data packet, saving bandwidth and ensuring accurate reconstruction on the receiving end.

2. Introduction to PHP pack() function

pack() is a built-in PHP function used to convert data into a binary string, commonly used in network communication, file storage, and more. Its basic syntax is as follows:

string pack(string $format, mixed ...$values)

The $format parameter is a format control string that specifies the types and order of the following values, such as:

f: 32-bit floating point (little-endian)
d: 64-bit floating point
N: 32-bit unsigned integer (big-endian)
V: 32-bit unsigned integer (little-endian)

3. Packing GPS coordinates and timestamp

For compatibility and data compactness, assume we use:

Latitude: float (4 bytes, little-endian)
Longitude: float (4 bytes, little-endian)
Timestamp: uint32 (4 bytes, little-endian)

We can pack these data using the following code:

<?php
// GPS data
$latitude = 31.2304;
$longitude = 121.4737;
$timestamp = time(); // current timestamp
<p>// Pack using little-endian format (latitude: float, longitude: float, timestamp: uint32)<br>
$binaryData = pack('ffV', $latitude, $longitude, $timestamp);</p>
<p>// Optional: convert packed binary data to hex for easier viewing<br>
$hexData = bin2hex($binaryData);</p>
<p>echo "Packed binary data: $hexData\n";<br>
?><br>

This code outputs something like:

Packed binary data: 9ad4fc3f7089514270128c66

Each segment corresponds to the packed latitude, longitude, and timestamp. You can use unpack() on the server side to parse it back:

<?php
$receivedData = $binaryData; // assuming data received from the device
$unpacked = unpack('flatitude/flongitude/Vtimestamp', $receivedData);
<p>print_r($unpacked);<br>
?><br>

4. Device transmission recommendations

The device can use the same binary structure to pack data and upload it to the server via MQTT, HTTP POST, or UDP. For example:

$url = "https://m66.net/api/receive.php";
$data = $binaryData;
<p>$options = [<br>
'http' => [<br>
'method' => "POST",<br>
'header' => "Content-Type: application/octet-stream\r\n",<br>
'content' => $data<br>
]<br>
];<br>
$context = stream_context_create($options);<br>
$response = file_get_contents($url, false, $context);</p>
<p>echo "Server response: $response\n";<br>

The server reads the raw binary data through php://input and parses it accordingly.

5. Notes

The floating-point format used in pack() may slightly differ across platforms, so it's important to consistently use either little-endian or big-endian and document the format.
Ensure the device and server use the same byte order.
If the transmitted data structure is complex, consider adding a header with identifiers such as type or version number.

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