Surface a better warning when the LDAP extension is not installed

Summary:
Fixes T3347. We can't really do this one as a config thing since we don't know if the user wants to use LDAP.

Instead, just give them a better message than they otherwise get when they try to install/configure/use LDAP.

Test Plan: Faked it and got a reasonable message.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3347

Differential Revision: https://secure.phabricator.com/D10260

src/applications/auth/management/PhabricatorAuthManagementLDAPWorkflow.php

@@ -15,6 +15,8 @@ final class PhabricatorAuthManagementLDAPWorkflow
     $console = PhutilConsole::getConsole();
     $console->getServer()->setEnableLog(true);
 
+    PhabricatorLDAPAuthProvider::assertLDAPExtensionInstalled();
+
     $provider = PhabricatorLDAPAuthProvider::getLDAPProvider();
     if (!$provider) {
       $console->writeOut(

src/applications/auth/provider/PhabricatorLDAPAuthProvider.php

@@ -251,12 +251,26 @@ final class PhabricatorLDAPAuthProvider extends PhabricatorAuthProvider {
     return array($errors, $issues, $values);
   }
 
+  public static function assertLDAPExtensionInstalled() {
+    if (!function_exists('ldap_bind')) {
+      throw new Exception(
+        pht(
+          'Before you can set up or use LDAP, you need to install the PHP '.
+          'LDAP extension. It is not currently installed, so PHP can not '.
+          'talk to LDAP. Usually you can install it with '.
+          '`yum install php-ldap`, `apt-get install php5-ldap`, or a '.
+          'similar package manager command.'));
+    }
+  }
+
   public function extendEditForm(
     AphrontRequest $request,
     AphrontFormView $form,
     array $values,
     array $issues) {
 
+    self::assertLDAPExtensionInstalled();
+
     $labels = $this->getPropertyLabels();
 
     $captions = array(