replace jessie to stretch, using name PHP7 instead of PHP

tools/dockerfile/distribtest/php7_stretch_x64/Dockerfile

@@ -0,0 +1,21 @@
+# Copyright 2020 gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+FROM debian:stretch
+
+RUN apt-get update && apt-get install -y php php-dev php-pear wget
+
+RUN wget https://phar.phpunit.de/phpunit-5.7.27.phar && \
+  mv phpunit-5.7.27.phar /usr/local/bin/phpunit && \
+  chmod +x /usr/local/bin/phpunit

tools/run_tests/artifacts/distribtest_targets.py

@@ -227,11 +227,11 @@ class RubyDistribTest(object):
         return self.name
 
 
-class PHPDistribTest(object):
+class PHP7DistribTest(object):
     """Tests PHP package"""
 
     def __init__(self, platform, arch, docker_suffix=None):
-        self.name = 'php_%s_%s_%s' % (platform, arch, docker_suffix)
+        self.name = 'php7_%s_%s_%s' % (platform, arch, docker_suffix)
         self.platform = platform
         self.arch = arch
         self.docker_suffix = docker_suffix
@@ -246,7 +246,7 @@ class PHPDistribTest(object):
         if self.platform == 'linux':
             return create_docker_jobspec(
                 self.name,
-                'tools/dockerfile/distribtest/php_%s_%s' %
+                'tools/dockerfile/distribtest/php7_%s_%s' %
                 (self.docker_suffix, self.arch),
                 'test/distrib/php/run_distrib_test.sh',
                 copy_rel_path='test/distrib')
@@ -374,6 +374,6 @@ def targets():
         RubyDistribTest('linux', 'x64', 'ubuntu1604'),
         RubyDistribTest('linux', 'x64', 'ubuntu1804'),
         # PHP
-        PHPDistribTest('linux', 'x64', 'jessie'),
-        PHPDistribTest('macos', 'x64'),
+        PHP7DistribTest('linux', 'x64', 'stretch'),
+        PHP7DistribTest('macos', 'x64'),
     ]