add check to see if file exists
diff --git a/tools/run_tests/run_interop_tests.py b/tools/run_tests/run_interop_tests.py
index fe8050d..cecb73d 100755
--- a/tools/run_tests/run_interop_tests.py
+++ b/tools/run_tests/run_interop_tests.py
@@ -481,7 +481,7 @@
# This env variable is used to get around the github rate limit
# error when running the PHP `composer install` command
# TODO(stanleycheung): find a more elegant way to do this
- if language.safename == 'php':
+ if language.safename == 'php' and os.path.exists('/var/local/.composer/auth.json'):
env['BUILD_INTEROP_DOCKER_EXTRA_ARGS'] = \
"-v /var/local/.composer/auth.json:/root/.composer/auth.json:ro"
build_job = jobset.JobSpec(