Deprecated: mysql_connect() mysql extension is deprecated and will be removed in the future

I have this error Deprecated: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead in - how to solve it?
My connect.php page is:

Example

<?php

$database="my_database_name";
$mysql_user = "my_mysql_user";
$mysql_password = "1234";
$mysql_host = "localhost";
$mysql_table_prefix = "";

$success = mysql_connect ($mysql_host, $mysql_user, $mysql_password);
if (!$success)
die ("<b>Cannot connect to database, check if username, password and host are correct.</b>");
$success = mysql_select_db ($database);
if (!$success) {
print "<b>Cannot choose database, check if database name is correct.";
die();
}
?>

I have this error deprecated mysql_connect() mysql extension is deprecated how to solve it?

deprecated mysql_connect Deprecated: mysql_connect() mysql extension is deprecated and will be removed - php mysqli

1274. PHP MySQLI functions to handle MySql database
PDO layer in PHP to mange MySQL database with different data handling commands in PHP
https://www.plus2net.com/php_tutorial/mysqli.php

3284. PHP mysqli_query() Function - mysqli query, sql tutorial
PHP mysqli_query() Function - mysqli query sql tutorial
https://www.agernic.com/sql-tutorial/php-mysqli_query-function-database-mysqli.html

1265. mysqli_fetch_field_direct to get field meta data
mysqli_fetch_field_direct to get field meta data in PHP MYSQLI
https://www.plus2net.com/php_tutorial/mysqli_fetch_field_direct.php

3285. PHP mysqli_connect () Function, sql tutorial
PHP mysqli_connect () Function sql tutorial
https://www.agernic.com/sql-tutorial/php-mysqli-connect-function.html

3271. How to prevent sql injection in php mysqli, php mysqli
How to prevent sql injection in php mysqli php mysqli
https://www.agernic.com/php-mysqli/how-to-prevent-sql-injection-in-php-mysqli.html

3273. Uncaught Error: Call to undefined function MYSQL_NUM_ROWS() in, php mysqli
Uncaught Error: Call to undefined function MYSQL_NUM_ROWS() in php mysqli
https://www.agernic.com/php-mysqli/uncaught-error-call-to-undefined-function-mysql_num_rows().html