Brief Introduction to PHP eval () function definition and Usage _php instance

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.

Returns NULL if the return statement is not invoked in the code string. If there is a parse error in the code, the eval () function returns FALSE.

Grammar

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

Tips and comments

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

Note: This function is useful for storing code in database text fields for future calculations.

Example

<?php
$string = "Beautiful";
$time = "Winter";
 
$str = ' 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 defined dynamically according to the configuration of the system, and the specific code fragment 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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