On the definition and usage of PHP eval () function, a brief talk on eval_php tutorial

On the definition and usage of PHP eval () function, on 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 no return statement is called in the code string, NULL is returned. If there is a parsing error in the code, the eval () function returns FALSE.

Grammar

Parameters	Description
Phpcode	Necessary. Specifies the PHP code to be computed.

Hints and Notes

Note: The return statement terminates the calculation of the string immediately.

Note: This function is useful for code storage that is made available for future calculations in a database text field.

Example

<?php$string = "Beautiful"; $time = "Winter"; $str = ' This is a $string $time morning! '; 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 configuration of the system, with the following 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 (' C i_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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