diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 00000000..82531282
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1 @@
+/.git* export-ignore
diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
new file mode 100644
index 00000000..1325301f
--- /dev/null
+++ b/.github/PULL_REQUEST_TEMPLATE.md
@@ -0,0 +1,8 @@
+Please do not submit any Pull Requests here. They will be closed.
+---
+
+Please submit your PR here instead:
+https://github.com/symfony/polyfill
+
+This repository is what we call a "subtree split": a read-only subset of that main repository.
+We're looking forward to your PR there!
diff --git a/.github/workflows/check-subtree-split.yml b/.github/workflows/check-subtree-split.yml
new file mode 100644
index 00000000..efc14326
--- /dev/null
+++ b/.github/workflows/check-subtree-split.yml
@@ -0,0 +1,33 @@
+name: Check subtree split
+on:
+  pull_request_target:
+jobs:
+  close-pull-request:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Close pull request
+      uses: actions/github-script@v6
+      with:
+        script: |
+          if (context.repo.owner === "symfony") {
+            github.rest.issues.createComment({
+              owner: "symfony",
+              repo: context.repo.repo,
+              issue_number: context.issue.number,
+              body: `
+          Thanks for your Pull Request! We love contributions.
+
+          However, you should instead open your PR on the main repository:
+          https://github.com/symfony/polyfill
+
+          This repository is what we call a "subtree split": a read-only subset of that main repository.
+          We're looking forward to your PR there!
+          `
+            });
+            github.rest.pulls.update({
+              owner: "symfony",
+              repo: context.repo.repo,
+              pull_number: context.issue.number,
+              state: "closed"
+            });
+          }