Add LiveUser->get_primary_institution(). Bug 1478374

and fix error of using [] with a function in PHP 5.3

Change-Id: I0199d6495af7f9df3c1935d6581a13f087b6dea1

htdocs/auth/user.php

@@ -1714,6 +1714,15 @@ class LiveUser extends User {
         return $this->defaults[$key];
     }
 
+    /**
+     * Gets the primary institution
+     * @return institution id or 'mahara' if not set
+     */
+    public function get_primary_institution() {
+        $institutions = array_keys($this->get('institutions'));
+        return !empty($institutions[0]) ? $institutions[0] : 'mahara';
+    }
+
     /**
      * Sets the property keyed by $key
      */

htdocs/lib/web.php

@@ -2898,7 +2898,7 @@ function main_nav() {
 
     $cachemenu = false;
     // Get the first institution
-    $institution = !empty(array_keys($USER->get('institutions'))[0]) ? array_keys($USER->get('institutions'))[0] : 'mahara';
+    $institution = $USER->get_primary_institution();
     $menutype = '';
     if (in_admin_section()) {
         global $USER, $SESSION;