原因
Whenever you specify "localhost" or "localhost:port" as server, the MySQL client library will override this and try to connect to a local socket (named pipe on Windows). If you want to use TCP/IP, use "127.0.0.1" instead of "localhost". If the MySQL client library tries to connect to the wrong local socket, you should set the correct path as in your PHP configuration and leave the server field blank.
localhost 使用的 Linux socket，127.0.0.1 使用的是 tcp/ip
为什么我使用 localhost 一直没出问题
因为你的本机中只有一个 mysql 进程， 如果你有一个 node1 运行在 3306， 有一个 node2 运行在 3307
mysql -u root -h localhost -P 3306
mysql -u root -h localhost -P 3307
都会连接到同一个 mysql 进程， 因为 localhost 使用 Linux socket， 所以 -P 字段直接被忽略了， 等价于
mysql -u root -h localhost
mysql -u root -h localhost
而 -h 默认是 localhost， 又等价于
mysql -u root
mysql -u root
为了避免这种情况（比如你在本地开发只有一个 mysql 进程，线上或者 qa 环境有多个 mysql 进程）最好的方式就是使用 IP
mysql -u root -h 127.0.0.1 -P 3307
第 1 条附言 · 3 小时 55 分钟前
使用strace查看：
strace mysql -h localhost -P 3306 -u root &> /tmp/strace_mysql.txt
结果：
execve("/opt/app/mysql/bin/mysql", ["mysql", "-h", "localhost", "-P", "3306", "-u", "root"], [/* 25 vars */]) = 0
......
socket(AF_LOCAL, SOCK_STREAM, 0) = 3
connect(3, {sa_family=AF_LOCAL, sun_path="/tmp/mysql.sock"}, 110) = 0 // 这里
setsockopt(3, SOL_TCP, TCP_NODELAY, [1], 4) = -1 EOPNOTSUPP (Operation not supported)
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
第 2 条附言 · 3 小时 52 分钟前
使用ip地址的strace
strace mysql -h 127.0.0.1 -P 3306 -u root &> /tmp/strace_mysql_use_ip.txt
execve("/opt/app/mysql/bin/mysql", ["mysql", "-h", "127.0.0.1", "-P", "3306", "-u", "root"], [/* 25 vars */]) = 0
....
socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(3306), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
setsockopt(3, SOL_TCP, TCP_NODELAY, [1], 4) = 0
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0