Summary of usage of php's eval function

$string = "beautiful";
$time = "winter";
$str = 'This is a $string $time morning!';
echo $str. "";
eval("$str = "$str";");
echo $str;
?>

output: This is a $string $time morning! This is a beautiful morning winter!

The

eval() function is also used in the CodeIgniter framework. In the /system/database/DB.php file, a class CI_DB is dynamically defined according to the system configuration. The specific code snippet:

if ( ! isset($active_record) OR $active_record == TRUE)
{
require_once(BASEPATH.'database/DB_active_rec.php');
if ( ! class_exists('CI_DB'))
{
eval('class CI_DB extends CI_DB_active_record { }');
}
}
else
{
if ( ! class_exists('CI_DB'))
{
eval('class CI_DB extends CI_DB_driver { }');
}
}
require_once(BASEPATH.'database/drivers/'.$params[' dbdriver'].'/'.$params['dbdriver'].'_driver.php');
// Instantiate the DB adapter
$driver = 'CI_DB_'.$params['dbdriver'].'_driver';
$DB = new $driver($params);

This function can substitute the variable value in the string and is usually used to process database data. The parameter code_str is the string to be processed. Note: The string to be processed must conform to PHP's string format and must have a semicolon at the end. The string processed using this function will be continued until the end of the PHP program.