diff --git a/.github/workflows/build.yaml b/.github/workflows/build.yaml
index 998e7c8a..24eb51a9 100644
--- a/.github/workflows/build.yaml
+++ b/.github/workflows/build.yaml
@@ -8,32 +8,19 @@ on:
   push:
 
 jobs:
-  build:
-    name: 'PHP ${{ matrix.php.branch }}'
+  prepare-ci:
+    name: 'Prepare actions'
     runs-on: ubuntu-20.04
-    strategy:
-      matrix:
-        php:
-          - branch: '8.1'
-          - branch: '8.0'
-          - branch: '7.4'
-          - branch: '7.3'
-          - branch: '7.2'
-          - branch: '7.1'
-          - branch: '7.0'
-          - branch: '5.6'
-      # We want to fix failures individually.
-      fail-fast: false
     steps:
       - uses: actions/checkout@v2
 
       - name: Install Nix
         uses: cachix/install-nix-action@v14
 
-      - name: Set up Nix cache
-        uses: cachix/cachix-action@v10
-        with:
-          name: fossar
-          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-
-      - run: nix-shell run-flake-checks.nix --argstr branch "${{ matrix.php.branch }}"
+      - name: Generate actions
+        run: nix-shell .github/workflows/workflow-from-flake-checks.nix
+  test:
+    needs:
+      - prepare-ci
+    uses: ./.github/workflows/test.yaml
+    secrets: inherit
diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
new file mode 100644
index 00000000..ac2be719
--- /dev/null
+++ b/.github/workflows/test.yaml
@@ -0,0 +1,130 @@
+{
+  "jobs": {
+    "test": {
+      "name": "PHP ${{ matrix.php.branch }}",
+      "runs-on": "${{ matrix.os.image }}",
+      "steps": [
+        {
+          "uses": "actions/checkout@v2"
+        },
+        {
+          "name": "Install Nix",
+          "uses": "cachix/install-nix-action@v14"
+        },
+        {
+          "name": "Set up Nix cache",
+          "uses": "cachix/cachix-action@v10",
+          "with": {
+            "authToken": "${{ secrets.CACHIX_AUTH_TOKEN }}",
+            "name": "fossar"
+          }
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\",\"php80\",\"php81\"]'), matrix.php.attr) }}",
+          "name": "Check that composer PHAR works",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-composer-phar"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\",\"php80\",\"php81\"]'), matrix.php.attr) }}",
+          "name": "Build Imagick extension",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-imagick"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\"]'), matrix.php.attr) }}",
+          "name": "Build MySQL extension",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-mysql"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\",\"php80\",\"php81\"]'), matrix.php.attr) }}",
+          "name": "Validate php.extensions.mysqli default unix socket path",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-mysqli-socket-path"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\",\"php80\",\"php81\"]'), matrix.php.attr) }}",
+          "name": "Validate php.extensions.pdo_mysql default unix socket path",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-pdo_mysql-socket-path"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\",\"php80\",\"php81\"]'), matrix.php.attr) }}",
+          "name": "Build PHP",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-php"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\",\"php80\",\"php81\"]'), matrix.php.attr) }}",
+          "name": "Build Redis extension",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-redis"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\"]'), matrix.php.attr) }}",
+          "name": "Build Redis 3 extension",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-redis3"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\",\"php80\",\"php81\"]'), matrix.php.attr) }}",
+          "name": "Build Tidy extension",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-tidy"
+        },
+        {
+          "if": "${{ contains(fromJSON('[\"aarch64-darwin\",\"aarch64-linux\",\"i686-linux\",\"x86_64-darwin\",\"x86_64-linux\"]'), matrix.os.platform) && contains(fromJSON('[\"php56\",\"php70\",\"php71\",\"php72\",\"php73\",\"php74\",\"php80\",\"php81\"]'), matrix.php.attr) }}",
+          "name": "Build Xdebug extension",
+          "run": "nix-build -A outputs.checks.${{ matrix.os.platform }}.${{ matrix.php.attr }}-xdebug"
+        }
+      ],
+      "strategy": {
+        "fail-fast": false,
+        "matrix": {
+          "os": [
+            {
+              "image": "ubuntu-20.04",
+              "platform": "x86_64-linux"
+            }
+          ],
+          "php": [
+            {
+              "attr": "php56",
+              "branch": "5.6"
+            },
+            {
+              "attr": "php70",
+              "branch": "7.0"
+            },
+            {
+              "attr": "php71",
+              "branch": "7.1"
+            },
+            {
+              "attr": "php72",
+              "branch": "7.2"
+            },
+            {
+              "attr": "php73",
+              "branch": "7.3"
+            },
+            {
+              "attr": "php74",
+              "branch": "7.4"
+            },
+            {
+              "attr": "php80",
+              "branch": "8.0"
+            },
+            {
+              "attr": "php81",
+              "branch": "8.1"
+            }
+          ]
+        }
+      }
+    }
+  },
+  "name": "Test",
+  "on": {
+    "workflow_call": {
+      "secrets": {
+        "CACHIX_AUTH_TOKEN": {
+          "required": true
+        }
+      }
+    }
+  }
+}
diff --git a/.github/workflows/workflow-from-flake-checks.nix b/.github/workflows/workflow-from-flake-checks.nix
new file mode 100644
index 00000000..8d89b761
--- /dev/null
+++ b/.github/workflows/workflow-from-flake-checks.nix
@@ -0,0 +1,151 @@
+# Runs Nix flake checks individually with group markers for GitHub Actions.
+# Invoke with `nix-shell workflow-from-flake-checks.nix`
+
+let
+  self = import ../..;
+
+  pkgs = self.inputs.nixpkgs.legacyPackages.${builtins.currentSystem};
+  inherit (self.inputs.nixpkgs) lib;
+
+  checkAttrs =
+    lib.foldl'
+      (
+        acc:
+        {
+          name,
+          value,
+        }:
+
+        acc // {
+          "${name}" = {
+            inherit (value) description;
+            platforms = acc.${name}.platforms or {} // { "${value.platform}" = null; };
+            phps = acc.${name}.phps or {} // { "${value.php}" = null; };
+          };
+        }
+      )
+      {}
+      (
+        builtins.concatLists (
+          lib.mapAttrsToList
+            (
+              platform:
+              checks:
+
+              lib.mapAttrsToList
+                (
+                  attr:
+                  check:
+
+                  let
+                    match = builtins.match "(php[0-9]+)-(.+)" attr;
+                  in
+                  {
+                    name = builtins.head (builtins.tail match);
+                    value = {
+                      inherit (check) description;
+                      inherit platform;
+                      php = builtins.head match;
+                    };
+                  }
+                )
+                checks
+            )
+            self.outputs.checks
+        )
+      );
+
+  packages = self.outputs.packages.${builtins.currentSystem};
+  phpPackages = builtins.filter (name: builtins.match "php[0-9]+" name != null) (builtins.attrNames packages);
+
+  workflow = {
+    name = "Test";
+
+    on = {
+      workflow_call = {
+        secrets = {
+          CACHIX_AUTH_TOKEN = {
+            required = true;
+          };
+        };
+      };
+    };
+
+    jobs = {
+      test = {
+        name = "PHP \${{ matrix.php.branch }}";
+        runs-on = "\${{ matrix.os.image }}";
+        steps = [
+          {
+            uses = "actions/checkout@v2";
+          }
+          {
+            name = "Install Nix";
+            uses = "cachix/install-nix-action@v14";
+          }
+          {
+            name = "Set up Nix cache";
+            uses = "cachix/cachix-action@v10";
+            "with" = {
+              authToken = "\${{ secrets.CACHIX_AUTH_TOKEN }}";
+              name = "fossar";
+            };
+          }
+        ]
+        ++ lib.mapAttrsToList
+          (
+            attr:
+            check:
+
+            {
+              name = check.description;
+              "if" = "\${{ contains(fromJSON('${lib.escape [ "\\" "'" ] (builtins.toJSON (builtins.attrNames check.platforms))}'), matrix.os.platform) && contains(fromJSON('${lib.escape [ "\\" "'" ] (builtins.toJSON (builtins.attrNames check.phps))}'), matrix.php.attr) }}";
+              run = "nix-build -A outputs.checks.\${{ matrix.os.platform }}.\${{ matrix.php.attr }}-${attr}";
+            }
+          )
+          checkAttrs
+        ;
+        strategy = {
+          # We want to fix failures individually.
+          fail-fast = false;
+          matrix = {
+            os = [
+              {
+                image = "ubuntu-20.04";
+                platform = "x86_64-linux";
+              }
+            ];
+            php =
+              builtins.map
+                (
+                  phpAttr:
+
+                  {
+                    branch = lib.versions.majorMinor packages.${phpAttr}.version;
+                    attr = phpAttr;
+                  }
+                )
+                phpPackages;
+          };
+        };
+      };
+    };
+  };
+in
+
+pkgs.stdenv.mkDerivation {
+  name = "workflow-from-flake-checks";
+
+  buildCommand = ''
+    echo 'Please run `nix-shell workflow-from-flake-checks.nix`, `nix-build` cannot be used.' > /dev/stderr
+    exit 1
+  '';
+
+  shellHook =
+    ''
+      set -o errexit
+      echo ${lib.escapeShellArgs [ (builtins.toJSON workflow) ]} | ${pkgs.jq}/bin/jq . > .github/workflows/test.yaml
+      exit 0
+    ''
+    ;
+}
diff --git a/run-flake-checks.nix b/run-flake-checks.nix
deleted file mode 100644
index 199f3cbe..00000000
--- a/run-flake-checks.nix
+++ /dev/null
@@ -1,67 +0,0 @@
-# Runs Nix flake checks individually with group markers for GitHub Actions.
-# Invoke with `nix-shell run-flake-checks.nix --argstr branch "8.1"`
-{
-  # PHP attribute to run checks for. `null` for all checks.
-  branch ? null,
-}:
-
-let
-  self = import ./.;
-
-  pkgs = self.inputs.nixpkgs.legacyPackages.${builtins.currentSystem};
-  inherit (self.inputs.nixpkgs) lib;
-
-  checks = self.outputs.checks.${builtins.currentSystem};
-
-  phpName =
-    assert lib.assertMsg (builtins.match "[0-9]+.[0-9]+" branch != null) "Branch name “${builtins.toString branch}” does not match a version number.";
-
-    "php${lib.versions.major branch}${lib.versions.minor branch}";
-
-  relevantChecks =
-    if branch == null
-    then checks
-    else lib.filterAttrs (key: value: lib.hasPrefix "${phpName}-" key) checks;
-in
-
-assert lib.assertMsg (relevantChecks != { }) "No checks found for branch “${builtins.toString branch}”.";
-
-pkgs.stdenv.mkDerivation {
-  name = "run-flake-checks";
-
-  buildCommand = ''
-    echo 'Please run `nix-shell run-flake-checks.nix`, `nix-build` cannot be used.' > /dev/stderr
-    exit 1
-  '';
-
-  shellHook =
-    ''
-      set -o errexit
-    ''
-    + builtins.concatStringsSep
-      "\n"
-      (
-        lib.mapAttrsToList
-          (
-            name:
-            value:
-
-            let
-              description =
-                lib.optionalString (branch == null) "PHP ${value.phpBranch} – "
-                + value.description;
-            in
-            ''
-              echo "::group::${description}"
-              echo Run nix-build -A outputs.checks.${builtins.currentSystem}.${name}
-              nix-build --no-out-link -A outputs.checks.${builtins.currentSystem}.${name}
-              echo "::endgroup::"
-            ''
-          )
-          relevantChecks
-      )
-    + ''
-      exit 0
-    ''
-    ;
-}