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Hooks are WHMCS's extensibility system. They let you run custom PHP code at specific events — client signup, invoice creation, ticket reply, etc. — without modifying WHMCS's core files. This article covers the basic hook patterns that solve common reseller customization needs.

Why use hooks

• Survives upgrades. WHMCS upgrades replace core files. Hooks live in your includes/hooks/ directory, untouched.
• Compose with other extensions. Multiple hooks can fire at the same event without stepping on each other.
• Standard pattern. Anyone hiring a developer to customize WHMCS for you will write hooks.

Hook structure

A hook is a PHP file in includes/hooks/ that registers a function with WHMCS's event system:

<?php
// File: includes/hooks/my_first_hook.php
add_hook('ClientAdd', 1, function($vars) {
    // $vars contains the new client's data
    // Do something with it
    return [];
});

That's it. Drop the file in includes/hooks/, hit Save in your code editor, and the hook is active. WHMCS auto-discovers hooks at the path on each request.

Anatomy of a hook call

• Event name (e.g., ClientAdd) — when WHMCS fires the hook
• Priority (integer; lower runs first; default 1)
• Callback function — receives $vars array containing context
• Return value — usually empty array; some hook types let you modify behavior by returning specific structures

Common hook events

WHMCS has 200+ hook events. The most useful for a typical reseller:

Client lifecycle

• ClientAdd — client signs up
• ClientEdit — client info updated
• ClientLogin — client logs in
• ClientClose — client account closed

Service lifecycle

• AfterModuleCreate — cPanel account just created on your server
• AfterModuleSuspend — account suspended
• AfterModuleUnsuspend — account unsuspended
• AfterModuleTerminate — account terminated

Billing

• InvoiceCreated — new invoice generated
• InvoicePaid — invoice marked as paid
• InvoiceCancelled — invoice cancelled

Support

• TicketOpen — new ticket submitted
• TicketAdminReply — admin posts a reply
• TicketUserReply — client posts a reply

Cron / automation

• DailyCronJob — runs once per day
• AfterCronJob — runs after every 5-minute cron

Useful hook examples

Send a Slack notification on new signup

<?php
add_hook('ClientAdd', 1, function($vars) {
    $payload = json_encode([
        'text' => "New signup: {$vars['firstname']} {$vars['lastname']} ({$vars['email']})"
    ]);
    $ch = curl_init('https://hooks.slack.com/services/YOUR/WEBHOOK/URL');
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $payload);
    curl_setopt($ch, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_exec($ch);
    curl_close($ch);
    return [];
});

Email yourself when a high-value invoice is paid

<?php
add_hook('InvoicePaid', 1, function($vars) {
    if ($vars['amount'] > 100) {
        mail('you@yourdomain.com',
             "Big invoice paid: ${$vars['amount']}",
             "Invoice ID: {$vars['invoiceid']}
Client: {$vars['userid']}");
    }
    return [];
});

Add a custom welcome message to all new accounts

<?php
add_hook('AfterModuleCreate', 1, function($vars) {
    $client_email = WHMCSDatabaseCapsule::table('tblclients')
        ->where('id', $vars['userid'])
        ->value('email');

    mail($client_email,
         'Welcome to YourBrand',
         'Thanks for choosing us! Here are some getting-started resources...');
    return [];
});

Auto-tag tickets containing the word "urgent"

<?php
add_hook('TicketOpen', 1, function($vars) {
    if (stripos($vars['message'], 'urgent') !== false) {
        WHMCSDatabaseCapsule::table('tbltickets')
            ->where('id', $vars['ticketid'])
            ->update(['priority' => 'High']);
    }
    return [];
});

Database access in hooks

WHMCS exposes the Laravel Capsule database layer:

// SELECT
$client = WHMCSDatabaseCapsule::table('tblclients')->where('id', $vars['userid'])->first();

// UPDATE
WHMCSDatabaseCapsule::table('tblclients')->where('id', $vars['userid'])->update(['notes' => 'New note']);

// INSERT
WHMCSDatabaseCapsule::table('tbl_my_custom_table')->insert([
    'field1' => 'value1',
    'created' => date('Y-m-d H:i:s'),
]);

Avoid raw mysql_ functions or $query = "SELECT..." string-building — you'll create SQL injection vulnerabilities and your hook will break on WHMCS upgrades.

Logging from hooks

Don't use echo or var_dump — they break the page rendering. Instead:

logActivity("My hook did something interesting: " . print_r($vars, true), 0);

Logs appear in WHMCS Admin → Utilities → Logs → Activity Log.

Security considerations

• Never trust hook input directly. Validate and escape any data you pass to mail(), database queries, or external APIs.
• Use prepared statements. Capsule handles this automatically when you use the query builder.
• Don't expose sensitive data. Logging full client objects exposes PII in logs. Filter what you log.
• Use HTTPS for outbound webhooks. Slack, Discord, etc. all require HTTPS — make sure your curl calls use it.

Hook discovery and debugging

• WHMCS Admin → Utilities → Logs → Module Log shows hook execution and errors.
• Hook errors silently fail by default. Wrap your hook in try/catch and log exceptions.
• The full hook reference: developers.whmcs.com/hooks

What to put in hooks vs. not

Good fits:

• Triggering external APIs (Slack, Discord, custom CRM)
• Custom email notifications beyond WHMCS templates
• Auto-tagging or auto-categorizing tickets
• Custom analytics events
• Provisioning auxiliary services (DNS records, monitoring entries)

Not good fits:

• Modifying WHMCS's built-in workflows (use module hooks or custom modules instead)
• Heavy database operations (run them in a cron, not inline in user-facing requests)
• Anything time-sensitive that should fail visibly — hooks are silent on failure

Common pitfalls

• Hook file outside includes/hooks/. WHMCS auto-discovers hooks only in that directory.
• Wrong PHP version. Hooks must work with the PHP version your WHMCS is running on. Test locally before deploying.
• Slow external APIs blocking page loads. A signup flow that calls Slack synchronously will appear slow. Use curl_multi_* or background jobs for slow external calls.
• Forgotten hooks after disabling a feature. If you wrote a hook for a feature you no longer use, remove the file. Orphaned hooks add complexity for the next person.