PHPeval () function definition and usage _ php instance

The following is a brief introduction to the definition and usage of the PHPeval () function. I think this is quite good. now I will share it with you and give you a reference. Let's take a look at the eval () function to calculate 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 $str. "
"; eval("\$str = \"$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);

The definition and usage of the PHP eval () function is all the content that I have shared with you in this article. I hope you can give us a reference and support for me.