Accommodate PHP 7.4 changes to certain "preg_match_all()" calls

Summary:
Ref T13518. The result format of this call changed in PHP 7.4, which causes us to emit "-1" matches because "-1" survives `array_filter()`.

Filter results in a way that should survive both result formats.

Test Plan: Ran `arc unit --everything` under PHP 7.4.

Maniphest Tasks: T13518

Differential Revision: https://secure.phabricator.com/D21173

src/parser/xhpast/api/XHPASTNode.php

@@ -163,7 +163,26 @@ final class XHPASTNode extends AASTNode {
     $re = '/\\\\.|(\$|\{\$|\${)([a-z_\x7F-\xFF][a-z0-9_\x7F-\xFF]*)/i';
     $matches = null;
     preg_match_all($re, $value, $matches, PREG_OFFSET_CAPTURE);
-    return ipull(array_filter($matches[2]), 0, 1);
+
+    // NOTE: The result format for this construction changed in PHP 7.4.
+    // See T13518.
+
+    $names = $matches[2];
+    foreach ($names as $name_idx => $name_match) {
+      if ($name_match === '') {
+        unset($names[$name_idx]);
+        continue;
+      }
+
+      if ($name_match[1] === -1) {
+        unset($names[$name_idx]);
+        continue;
+      }
+    }
+
+    $names = ipull($names, 0, 1);
+
+    return $names;
   }
 
   public function getStringLiteralValue() {