Fix + test

src/Peachpie.Library/pcre.cs

@@ -981,7 +981,8 @@ namespace Pchp.Library
             {
                 var g = groups[i];
                 var value = (g.Success || !unmatchedAsNull) ? g.Value : null;
-                var item = NewArrayItem(value, g.Index, offsetCapture);
+                int index = g.Success ? g.Index : -1;
+                var item = NewArrayItem(value, index, offsetCapture);
 
                 // All groups should be named.
                 if (g.IsNamedGroup)

tests/pcre/preg_match_006.php

@@ -0,0 +1,8 @@
+<?php
+namespace pcre\preg_match_006;
+
+preg_match('/(?P<fooname>foo)?(bar)/', 'bar', $matches, PREG_OFFSET_CAPTURE);
+
+$is_fooname = isset( $matches['fooname'] ) && -1 !== $matches['fooname'][1];
+
+echo "is_fooname:" . ($is_fooname ? "true" : "false") . "\n"; //is_fooname:false