Problem description
I just started to learn PHP, and the system environment is Ubuntu+PHP7.0+Mysql5.7+Apache2.
running a database connection test sample error:

[client 127.0.0.1:37496] PHP Fatal error:  Uncaught Error: Call to undefined function mysql_connect() in /var/www/html/test.php:2\nStack trace:\n#0 {main}\n  thrown in /var/www/html/test.php on line 2

The sample code is:

<?PHP
    $conn=mysql_connect("localhost","root","root");
    if($conn){
        echo"ok";
    }else{
        echo"error";    
    }
?>

The solution
The mysql_connect() function has been deprecated since PHP5.0. In PHP 7.0 it has been deprecated and replaced with this function:

mysqli_connect();

Usage is:

$con=mysqli_connect("localhost","my_user","my_password","my_db");

The description of the official connection: http://php.net/manual/en/function.mysqli-connect.php
the correct test code:

<?PHP
    $conn=mysqli_connect("localhost","root","root");
    if($conn){
        echo"ok";
    }else{
        echo"error";    
    }
?>

conclusion

in the Ubuntu + PHP7.0 + Mysql5.7 + Apache2 system under the environment of the wrong, because the mysql_connect () function has been deprecated, when following outdated tutorial learning may encounter this error. (note: if it is a Windows system, are more likely to be Apache2 not enable mysql, details on baidu) when running the above test code, interface without any reaction, the error is in the log to consult, log directory in the/var/log/Apache2/error log “.