diff --git a/lib/DebugHtml.php b/lib/DebugHtml.php
index ce2b8361..0f5af241 100644
--- a/lib/DebugHtml.php
+++ b/lib/DebugHtml.php
@@ -39,73 +39,75 @@ class Debug
     public static function printDebugTable($sHeading, $aVar)
     {
         echo '<b>'.$sHeading.":</b>\n";
-        echo '<table border="1">';
+        echo "<table border='1'>\n";
         if (!empty($aVar)) {
-            echo '<tr>';
+            echo "  <tr>\n";
             $aKeys = array();
             $aInfo = reset($aVar);
             if (!is_array($aInfo)) {
                 $aInfo = $aInfo->debugInfo();
             }
             foreach ($aInfo as $sKey => $mVal) {
-                echo '<th><small>'.$sKey.'</small></th>';
+                echo '    <th><small>'.$sKey.'</small></th>'."\n";
                 $aKeys[] = $sKey;
             }
-            echo '</tr>';
+            echo "  </tr>\n";
             foreach ($aVar as $oRow) {
                 $aInfo = $oRow;
                 if (!is_array($oRow)) {
                     $aInfo = $oRow->debugInfo();
                 }
-                echo '<tr>';
+                echo "  <tr>\n";
                 foreach ($aKeys as $sKey) {
-                    echo '<td><pre>';
+                    echo '    <td><pre>';
                     if (isset($aInfo[$sKey])) {
                         Debug::outputVar($aInfo[$sKey], '');
                     }
-                    echo '</pre></td>';
+                    echo '</pre></td>'."\n";
                 }
-                echo '<tr>';
+                echo "  </tr>\n";
             }
         }
-        echo '</table>';
+        echo "</table>\n";
     }
 
     public static function printGroupTable($sHeading, $aVar)
     {
         echo '<b>'.$sHeading.":</b>\n";
-        echo '<table border="1">';
+        echo "<table border='1'>\n";
         if (!empty($aVar)) {
-            echo '<tr><th><small>Group</small></th>';
+            echo "  <tr>\n";
+            echo '    <th><small>Group</small></th>'."\n";
             $aKeys = array();
-            $aInfo = reset(reset($aVar));
+            $aInfo = reset($aVar)[0];
             if (!is_array($aInfo)) {
                 $aInfo = $aInfo->debugInfo();
             }
             foreach ($aInfo as $sKey => $mVal) {
-                echo '<th><small>'.$sKey.'</small></th>';
+                echo '    <th><small>'.$sKey.'</small></th>'."\n";
                 $aKeys[] = $sKey;
             }
-            echo '</tr>';
+            echo "  </tr>\n";
             foreach ($aVar as $sGrpKey => $aGroup) {
                 foreach ($aGroup as $oRow) {
                     $aInfo = $oRow;
                     if (!is_array($oRow)) {
                         $aInfo = $oRow->debugInfo();
                     }
-                    echo '<tr><td><pre>'.$sGrpKey.'</pre></td>';
+                    echo "  <tr>\n";
+                    echo '    <td><pre>'.$sGrpKey.'</pre></td>'."\n";
                     foreach ($aKeys as $sKey) {
-                        echo '<td><pre>';
+                        echo '    <td><pre>';
                         if (!empty($aInfo[$sKey])) {
                             Debug::outputVar($aInfo[$sKey], '');
                         }
-                        echo '</pre></td>';
+                        echo '</pre></td>'."\n";
                     }
-                    echo '<tr>';
+                    echo "  </tr>\n";
                 }
             }
         }
-        echo '</table>';
+        echo "</table>\n";
     }
 
     public static function printSQL($sSQL)
@@ -128,9 +130,9 @@ class Debug
                 $sPre = "\n".$sPreNL;
             }
         } elseif (is_array($mVar) && isset($mVar['__debug_format'])) {
-            if (!empty($mVar[data])) {
+            if (!empty($mVar['data'])) {
                 $sPre = '';
-                foreach ($mVar[data] as $mValue) {
+                foreach ($mVar['data'] as $mValue) {
                     echo $sPre;
                     Debug::outputSimpleVar($mValue);
                     $sPre = ', ';
diff --git a/test/php/Nominatim/DebugTest.php b/test/php/Nominatim/DebugTest.php
new file mode 100644
index 00000000..7ed08122
--- /dev/null
+++ b/test/php/Nominatim/DebugTest.php
@@ -0,0 +1,197 @@
+<?php
+
+namespace Nominatim;
+
+use Exception;
+
+require_once('../../lib/DebugHtml.php');
+
+class DebugTest extends \PHPUnit_Framework_TestCase
+{
+    protected function setUp()
+    {
+        $this->oWithDebuginfo = $this->getMock(Geocode::class, array('debugInfo'));
+        $this->oWithDebuginfo->method('debugInfo')
+                  ->willReturn(array('key1' => 'val1', 'key2' => 'val2', 'key3' => 'val3'));
+
+        $this->oWithToString = $this->getMock(Geocode::class, array('__toString'));
+        $this->oWithToString->method('__toString')->willReturn('me as string');
+    }
+
+    public function testPrintVar()
+    {
+        $this->expectOutputString(<<<EOT
+<pre><b>Var0:</b>  </pre>
+<pre><b>Var1:</b>  <i>True</i></pre>
+<pre><b>Var2:</b>  <i>False</i></pre>
+<pre><b>Var3:</b>  0</pre>
+<pre><b>Var4:</b>  'String'</pre>
+<pre><b>Var5:</b>  0 => 'one'
+       1 => 'two'
+       2 => 'three'</pre>
+<pre><b>Var6:</b>  'key' => 'value'
+       'key2' => 'value2'</pre>
+<pre><b>Var7:</b>  me as string</pre>
+<pre><b>Var8:</b>  'value', 'value2'</pre>
+
+EOT
+        );
+    
+        Debug::printVar('Var0', null);
+        Debug::printVar('Var1', true);
+        Debug::printVar('Var2', false);
+        Debug::printVar('Var3', 0);
+        Debug::printVar('Var4', 'String');
+        Debug::printVar('Var5', array('one', 'two', 'three'));
+        Debug::printVar('Var6', array('key' => 'value', 'key2' => 'value2'));
+        Debug::printVar('Var7', $this->oWithToString);
+        Debug::printVar('Var8', Debug::fmtArrayVals(array('key' => 'value', 'key2' => 'value2')));
+    }
+
+
+    public function testDebugArray()
+    {
+        $this->expectOutputString(<<<EOT
+<pre><b>Arr0:</b>  'null'</pre>
+<pre><b>Arr1:</b>  'key1' => 'val1'
+       'key2' => 'val2'
+       'key3' => 'val3'</pre>
+
+EOT
+        );
+    
+        Debug::printDebugArray('Arr0', null);
+        Debug::printDebugArray('Arr1', $this->oWithDebuginfo);
+    }
+
+
+    public function testPrintDebugTable()
+    {
+        $this->expectOutputString(<<<EOT
+<b>Table1:</b>
+<table border='1'>
+</table>
+<b>Table2:</b>
+<table border='1'>
+</table>
+<b>Table3:</b>
+<table border='1'>
+  <tr>
+    <th><small>0</small></th>
+    <th><small>1</small></th>
+  </tr>
+  <tr>
+    <td><pre>'one'</pre></td>
+    <td><pre>'two'</pre></td>
+  </tr>
+  <tr>
+    <td><pre>'three'</pre></td>
+    <td><pre>'four'</pre></td>
+  </tr>
+</table>
+<b>Table4:</b>
+<table border='1'>
+  <tr>
+    <th><small>key1</small></th>
+    <th><small>key2</small></th>
+    <th><small>key3</small></th>
+  </tr>
+  <tr>
+    <td><pre>'val1'</pre></td>
+    <td><pre>'val2'</pre></td>
+    <td><pre>'val3'</pre></td>
+  </tr>
+</table>
+
+EOT
+        );
+    
+        Debug::printDebugTable('Table1', null);
+
+        Debug::printDebugTable('Table2', array());
+
+        // Numeric headers
+        Debug::printDebugTable('Table3', array(array('one', 'two'), array('three', 'four')));
+
+        // Associate array
+        Debug::printDebugTable('Table4', array($this->oWithDebuginfo));
+    }
+
+    public function testPrintGroupTable()
+    {
+        $this->expectOutputString(<<<EOT
+<b>Table1:</b>
+<table border='1'>
+</table>
+<b>Table2:</b>
+<table border='1'>
+</table>
+<b>Table3:</b>
+<table border='1'>
+  <tr>
+    <th><small>Group</small></th>
+    <th><small>key1</small></th>
+    <th><small>key2</small></th>
+  </tr>
+  <tr>
+    <td><pre>group1</pre></td>
+    <td><pre>'val1'</pre></td>
+    <td><pre>'val2'</pre></td>
+  </tr>
+  <tr>
+    <td><pre>group1</pre></td>
+    <td><pre>'one'</pre></td>
+    <td><pre>'two'</pre></td>
+  </tr>
+  <tr>
+    <td><pre>group2</pre></td>
+    <td><pre>'val1'</pre></td>
+    <td><pre>'val2'</pre></td>
+  </tr>
+</table>
+<b>Table4:</b>
+<table border='1'>
+  <tr>
+    <th><small>Group</small></th>
+    <th><small>key1</small></th>
+    <th><small>key2</small></th>
+    <th><small>key3</small></th>
+  </tr>
+  <tr>
+    <td><pre>group1</pre></td>
+    <td><pre>'val1'</pre></td>
+    <td><pre>'val2'</pre></td>
+    <td><pre>'val3'</pre></td>
+  </tr>
+  <tr>
+    <td><pre>group1</pre></td>
+    <td><pre>'val1'</pre></td>
+    <td><pre>'val2'</pre></td>
+    <td><pre>'val3'</pre></td>
+  </tr>
+</table>
+
+EOT
+        );
+    
+        Debug::printGroupTable('Table1', null);
+        Debug::printGroupTable('Table2', array());
+
+        // header are taken from first group item, thus no key3 gets printed
+        $aGroups = array(
+            'group1' => array(
+                array('key1' => 'val1', 'key2' => 'val2'),
+                array('key1' => 'one', 'key2' => 'two', 'unknown' => 1),
+            ),
+            'group2' => array(
+                array('key1' => 'val1', 'key2' => 'val2', 'key3' => 'val3'),
+            )
+        );
+        Debug::printGroupTable('Table3', $aGroups);
+
+        $aGroups = array(
+            'group1' => array($this->oWithDebuginfo, $this->oWithDebuginfo),
+        );
+        Debug::printGroupTable('Table4', $aGroups);
+    }
+}