Implement Two-Way SMS with PHP

This article explores the intricacies of building two-way SMS applications using PHP, focusing on the complexities beyond simpler one-way systems. We'll cover the lifecycle, implementation details, and crucial considerations for choosing an SMS gateway.

Key Differences and Lifecycle:

Unlike one-way SMS (e.g., Gmail notifications), two-way SMS allows users to initiate conversations and receive application-generated responses. This involves a four-step lifecycle:

1. User to Gateway: The user sends a message to a designated shortcode or long number (provided by the SMS gateway).
2. Gateway to Server: The gateway forwards the message to your application server via HTTP GET or POST requests (often XML or SOAP).
3. Server to Gateway: Your PHP application processes the message, generates a response, and sends it back to the gateway using the gateway's API.
4. Gateway to User: The gateway delivers the response to the user.

Implementation with PHP and Clickatell (Example):

Choosing an SMS gateway (like Clickatell, Twilio, Nexmo, or Plivo) is crucial. Consider factors such as cost, reliability, delivery rates, API support, and customer service.

This example uses Clickatell to illustrate handling incoming messages via HTTP GET:

<?php
$fromNo = $_GET["from"];
$toNo = $_GET["to"];
$message = $_GET["text"];
$msgID = $_GET["moMsgId"];

// Process the user command and generate output...
?>

For XML POST requests, the process involves parsing the XML data:

<?php
$data = $_POST["data"];
$xmlDoc = new DOMDocument();
$xmlDoc->loadXML($data);
// Extract data from XML...
?>

Handling Long Messages (UDH):

Long messages are split into segments by the gateway. The User Data Header (UDH) contains information to reassemble these segments. The PHP code needs to check for UDH and reconstruct the complete message before processing. An example of handling UDH is shown below:

<?php
// ... (previous code) ...
$udh = $_GET["udh"];
$total = 1;
$count = 1;
if ($udh) {
    $tmp = str_split($udh, 2);
    $total = hexdec($tmp[4]);
    $count = hexdec($tmp[5]);
}

if ($count != $total) {
    // Save message part in database
} else if ($total != 1) {
    // Retrieve and combine previous parts from database
}
// ... (process the complete message) ...
?>

Sending Responses:

Sending responses involves using the gateway's API. Again, using Clickatell as an example:

<?php
$message = array(
    "to" => "+15551234567", // Recipient number
    "from" => "+15559876543", // Your registered number
    "text" => "Your response message",
    "api_id" => API_KEY, // Your API key
    // ... other parameters ...
);

// ... (cURL request to send the message) ...
?>

Conclusion:

Building a two-way SMS application with PHP requires careful consideration of the message lifecycle, handling of long messages using UDH, and the selection of a suitable SMS gateway. This guide provides a foundational understanding of the process and essential code examples to get you started. Remember to consult your chosen gateway's API documentation for specific details and parameters.

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