fixed php pdo errors for rds example, upgraded to amazon-linux-extra php7.4

example-projects/tug-of-war-in-the-clouds/aws-boto3-rds-db/start.py

@@ -159,7 +159,10 @@ userDataWebServer = ('#!/bin/bash\n'
                      '# essential tools\n'
                      'yum install -y joe htop git\n'
                      '# httpd and mysql client\n'
-                     'yum install -y httpd php php-mysql mariadb\n'
+                     'yum install -y httpd mariadb\n'
+                     '# fix php5.x PDO prob PDO::__construct(): Server sent charset (255) unknown to the client.\n'
+                     '# by using Amazons Linux 2 PHP extras\n'
+                     'amazon-linux-extras install -y php7.4\n'
                      '\n'
                      'service httpd start\n'
                      '\n'