Description:
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I was using the PHP 4.3.4 with MySQL 5.0.15. There was no problem with PHP when connecting to the MySQL using 

mysql_connect(($dbhost, $dbusername, $dbuserpass);

But, after installed the PHP 5.1.2, I can't connect to MySQL using the same command:

mysql_connect(($dbhost, $dbusername, $dbuserpass);

which would prompt an error:

"Call to undefined function mysql_connect()"

Reproduce code:
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mysql_connect(($dbhost, $dbusername, $dbuserpass);

Expected result:
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Successful connection to MySQL 5.0.15

Actual result:
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Call to undefined function mysql_connect()in line 12