Definitions and usage of PHP eval () functions, and eval
The eval () function calculates the string according to the PHP code.
The string must be a valid PHP code and must end with a semicolon.
If the return statement is not called in the code string, NULL is returned. If a parsing error exists in the Code, the eval () function returns false.
Syntax
eval(phpcode)
Parameters |
Description |
Phpcode |
Required. Specifies the PHP code to be calculated. |
Tips and comments
Note: The return statement immediately terminates string calculation.
Note: This function is useful for storing code in Database Text fields for future computation.
Example
<?php$string = "beautiful";$time = "winter"; $str = 'This is a $string $time morning!';echo $str. "<br />"; eval("\$str = \"$str\";");echo $str;?>
Output:
This is a $ string $ time morning!
This is a beautiful winter morning!
The eval () function is also useful in the CodeIgniter framework. In the/system/database/DB. php file, a class CI_DB is dynamically defined according to the system configuration,The code snippet is as follows:
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);
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