What the config_item function does not understand in the CodeIgniter source code

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if ( ! function_exists('get_config')){    /**     * Loads the main config.php file     *     * This function lets us grab the config file even if the Config class     * hasn't been instantiated yet     *     * @param    array     * @return    array     */    function &get_config(Array $replace = array())    {        static $config;        if (empty($config))        {            $file_path = APPPATH.'config/config.php';            $found = FALSE;            if (file_exists($file_path))            {                $found = TRUE;                require($file_path);            }            // Is the config file in the environment folder?            if (file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/config.php'))            {                require($file_path);            }            elseif ( ! $found)            {                set_status_header(503);                echo 'The configuration file does not exist.';                exit(3); // EXIT_CONFIG            }            // Does the $config array exist in the file?            if ( ! isset($config) OR ! is_array($config))            {                set_status_header(503);                echo 'Your config file does not appear to be formatted correctly.';                exit(3); // EXIT_CONFIG            }        }        // Are any values being dynamically added or replaced?        foreach ($replace as $key => $val)        {            $config[$key] = $val;        }        return $config;    }}// ------------------------------------------------------------------------if ( ! function_exists('config_item')){    /**     * Returns the specified config item     *     * @param    string     * @return    mixed     */    function config_item($item)    {        static $_config;        if (empty($_config))        {            // references cannot be directly assigned to static variables, so we use an array            $_config[0] =& get_config();        }        return isset($_config[0][$item]) ? $_config[0][$item] : NULL;    }}

$_config[0] =& get_config();Andfunction &get_config(Array $replace = array())Inside&The role of the symbol is not quite understood = used here&What is the special role of a symbol? = answer ~

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if ( ! function_exists('get_config')){    /**     * Loads the main config.php file     *     * This function lets us grab the config file even if the Config class     * hasn't been instantiated yet     *     * @param    array     * @return    array     */    function &get_config(Array $replace = array())    {        static $config;        if (empty($config))        {            $file_path = APPPATH.'config/config.php';            $found = FALSE;            if (file_exists($file_path))            {                $found = TRUE;                require($file_path);            }            // Is the config file in the environment folder?            if (file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/config.php'))            {                require($file_path);            }            elseif ( ! $found)            {                set_status_header(503);                echo 'The configuration file does not exist.';                exit(3); // EXIT_CONFIG            }            // Does the $config array exist in the file?            if ( ! isset($config) OR ! is_array($config))            {                set_status_header(503);                echo 'Your config file does not appear to be formatted correctly.';                exit(3); // EXIT_CONFIG            }        }        // Are any values being dynamically added or replaced?        foreach ($replace as $key => $val)        {            $config[$key] = $val;        }        return $config;    }}// ------------------------------------------------------------------------if ( ! function_exists('config_item')){    /**     * Returns the specified config item     *     * @param    string     * @return    mixed     */    function config_item($item)    {        static $_config;        if (empty($_config))        {            // references cannot be directly assigned to static variables, so we use an array            $_config[0] =& get_config();        }        return isset($_config[0][$item]) ? $_config[0][$item] : NULL;    }}

$_config[0] =& get_config();Andfunction &get_config(Array $replace = array())Inside&The role of the symbol is not quite understood = used here&What is the special role of a symbol? = answer ~

It can save memory space to a certain extent and indirectly modify the target value.
Reference delivery official documentation: http://www.php.net/manual/zh/language.references.pass.php
Reference back official documentation: http://php.net/manual/zh/language.references.return.php

Here is an extended example of function reference return.

Function & get_config () {static $ config = 0; $ config + = 1; echo sprintf ("config = % d \ n", $ config); return $ config ;} $ config_item = get_config (); $ config_item = 100; $ config_item = get_config (); $ config_item = & get_config (); // note the & $ config_item = 100; $ config_item = get_config (); // output config = 1 config = 2 config = 3 config = 101

Transfers references to save memory.

// They all point to the static global variable $ config's zval $ config1 = & get_config (); $ config2 = & get_config ();