diff --git a/platformio/project/config.py b/platformio/project/config.py
index f19473f8..3d92a5ae 100644
--- a/platformio/project/config.py
+++ b/platformio/project/config.py
@@ -316,10 +316,11 @@ class ProjectConfigBase:
             return value
 
         # legacy support for variables delclared without "${}"
+        legacy_vars = ["PROJECT_HASH"]
         stop = False
         while not stop:
             stop = True
-            for name in self.BUILTIN_VARS:
+            for name in legacy_vars:
                 x = value.find(f"${name}")
                 if x < 0 or value[x - 1] == "$":
                     continue
diff --git a/tests/project/test_config.py b/tests/project/test_config.py
index 6880dd7c..474033f3 100644
--- a/tests/project/test_config.py
+++ b/tests/project/test_config.py
@@ -354,10 +354,7 @@ def test_get_value(config):
         "--help",
     ]
     # test relative dir
-    assert config.get("platformio", "src_dir") == os.path.abspath(
-        os.path.join(os.getcwd(), "source")
-    )
-
+    assert config.get("platformio", "src_dir") == "source"
     # renamed option
     assert config.get("env:extra_1", "lib_install") == ["574"]
     assert config.get("env:extra_1", "lib_deps") == ["574"]
@@ -654,6 +651,7 @@ def test_nested_interpolation(tmp_path: Path):
         """
 [platformio]
 build_dir = /tmp/pio-$PROJECT_HASH
+data_dir = $PROJECT_DIR/assets
 
 [env:myenv]
 build_flags = -D UTIME=${UNIX_TIME}
@@ -667,9 +665,10 @@ test_testing_command =
     """
     )
     config = ProjectConfig(str(project_conf))
+    assert config.get("platformio", "data_dir") == "$PROJECT_DIR/assets"
+    assert config.get("env:myenv", "build_flags")[0][-10:].isdigit()
     testing_command = config.get("env:myenv", "test_testing_command")
     assert "$" not in " ".join(testing_command)
-    assert config.get("env:myenv", "build_flags")[0][-10:].isdigit()
 
 
 def test_extends_order(tmp_path: Path):