PHP built-in server on next unused port seq 8000 65000 defines the port range

next-port

#!/bin/sh
for port in $(seq 8000 65000); do echo -ne "\035" | telnet 127.0.0.1 $port > /dev/null 2>&1; [ $? -eq 1 ] && echo "$port" && break; done

start-servers

php -S localhost:`bin/next-port` &  # Listening on http://localhost:8000
php -S localhost:`bin/next-port` &  # Listening on http://localhost:8001
php -S localhost:`bin/next-port` &  # Listening on http://localhost:8002
php -S localhost:`bin/next-port` &  # Listening on http://localhost:8003