I was also stuck with the same problem of undefined MySQL_connect().I tried to make changes in PHP.ini file but it was giving me the same error. Then I came to this solution where I changed my code from depreciated php functions to new functions.
$con=mysqli_connect($host,$user,$password);
mysqli_select_db($con,dbname);
//To select the database
session_start(); //To start the session
$query=mysqli_query($con,your query);
//made query after establishing connection with database.
I hope this will help you . This solution is correctly working for me .
EDIT:
If you upgrade form old php you need to apt-get install php7.0-mysql