diff --git a/.github/workflows/php-security.yml b/.github/workflows/php-security.yml
index cd274fc..93c1da4 100644
--- a/.github/workflows/php-security.yml
+++ b/.github/workflows/php-security.yml
@@ -1,7 +1,16 @@
 # SPDX-License-Identifier: MPL-2.0
 name: PHP Security Check
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+
+# Estate guardrail: scope push to default branches so a PR fires once (not
+# push+PR), and cancel superseded runs. Safe — read-only PR-triggered check.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 permissions:
   contents: read