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Editorial Team
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Asked: 2026-05-14T03:15:18+00:00

Consider the following code: <?php $conn = mysql_connect(‘localhost’, ‘username’, ‘password’); mysql_select_db(‘database’, $conn); ?> This

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Consider the following code:

<?php

$conn = mysql_connect('localhost', 'username', 'password');
mysql_select_db('database', $conn);

?>

This works as expected, but how does PHP know what database connection to use when calling mysql_select_db() in the following example?

<?php

mysql_connect('localhost', 'username', 'password');
mysql_select_db('database');

?>

The PHP documentation states that "If the link identifier is not specified, the last link opened by mysql_connect() is assumed." (PHP: mysql_select_db())

Where is the last connection stored or retrieved from?

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1 Answer

  1. Editorial Team

    I suppose a link to the last opened connection is kept somewhere in memory, to make things easier (as we generally often use only one connection).

    Quickly going through the sources of ext/mysql :

    (All line numbers are in php_mysql.c — the version of the sources is a random snapshot of PHP 5.3.2-dev from a couple of weeks ago ; so, they might have changed a bit)

    • The user-space function called mysql_connect seems to correspond to the C-level function called php_mysql_do_connect (line 922)
    • The php_mysql_do_connect function calls php_mysql_set_default_link (line 832)
      • To store the last opened connection
    • There is also a function called php_mysql_get_default_link (line 908)
    • That php_mysql_get_default_link function is called by mysql_select_db, when there is no link passed to it (line 992)

    And php_mysql_set_default_link is calling this to store the default_link :

    MySG(default_link) = id;

    That MySG being a macro, defined like this (in php_mysql_structs.h) :

    #ifdef ZTS
# define MySG(v) TSRMG(mysql_globals_id, zend_mysql_globals *, v)
#else
# define MySG(v) (mysql_globals.v)
#endif

    Pretty much looks like a global variable to me 😉

    If you want, you can take a look at the sources yourself : ext/mysql/php_mysql.c and ext/mysql/php_mysql_structs.h.

    As I said, this has probably been modified a bit since the version in which I checked — which means the line numbers might not match exactly ; but the functions names are easy anough to understand, so you should be able to track down what calls what and where 🙂

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