diff --git a/.github/workflows/phar.yml b/.github/workflows/phar.yml
index 9ccc5ef72b4..2ca64b9fce3 100644
--- a/.github/workflows/phar.yml
+++ b/.github/workflows/phar.yml
@@ -2188,6 +2188,7 @@ jobs:
           docker run -d --name alpine-test \
             -v "$PWD:/work" -w /work \
             -e PHP_MINOR -e PHP_ZTS -e PHP_CACHE_DIR=/work/alpine-php-cache \
+            -e COMPOSER_CACHE_DIR=/work/composer-cache \
             "$ALPINE_IMAGE" sleep 7200
 
       # see turbo-compile
@@ -2208,6 +2209,17 @@ jobs:
       - name: "Trust the checkout despite the container/host uid mismatch"
         run: docker exec alpine-test git config --global --add safe.directory /work
 
+      # Without a cache, each of the 20 legs downloads every package from
+      # GitHub, and one failed download fails the leg. The differential
+      # tests ship nothing, so unlike turbo-compile-core-musl-arm64 they
+      # can trust a cache entry.
+      - name: "Restore the Composer cache"
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: "composer-cache"
+          key: "composer-musl-${{ hashFiles('composer.lock') }}"
+          restore-keys: "composer-musl-"
+
       - name: "Install Composer dependencies (verified composer)"
         run: |
           docker exec -i alpine-test sh -e <<'EOF'