php - How to connect two tables in database? -

i have database 2 tables:

table user:

[user_id (pk),username,password,email] 

and

table posts:

[post_id (pk),username_posted,post] 

i have created login-registration system first table, , want, when user logs in, post text pressing button. in second table, want store post_id, username of person posted, , of course post made. how can connect these 2 tables?

in relational database, rows in 2 tables "connected" storing common values in columns.

user: user_id  username  email -------  --------  -----       5  jack             6  jim              7  johnny                 post: post_id  post_username  post  -------  -------------  -------      11  jack           hi       12  jack           whassup      14  johnny         hey 

the rows in these 2 tables related ("connected") each other, values stored in username column , post_username column.

(we store value of user_id column user table in post table, same effect. we'll work schema have.)

as example of how add new row post table , "connect" existing row in user table:

 insert post (post_id, username, post)  values (15, 'jim', 'not much');