A problem with writing to the database after PHP gets the timestamp

Use $now_time=time(); in PHP to get the timestamp of the current time, and then pass $sql="UPDATE login_user SET first_logintime='".$now_time."' WHERE username='".$username."' AND pwd='".password."'; When writing to the database, I found that it could not be written. I thought that the timestamp could not be obtained. After I output it with echo after $now_time, I found that the writing was successful.When this happens? How can I write it to the database without outputting it?