PHP eval() function, phpeval function

PHP eval() function, phpeval function

Definition and usage

The eval() function evaluates strings according to PHP code.

The string must be valid PHP code and must end with a semicolon.

If no return statement is called in the code string, NULL is returned. If there are parsing errors in the code, the eval() function returns false.

Grammar

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eval(phpcode)

　

参数	描述
phpcode	必需。规定要计算的 PHP 代码。

Tips and Notes

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

Comments: This function is useful for storing code in a database text field for later calculations.

Example

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

输出：

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

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<br />eval() 函数在CodeIgniter框架里也有用到。在 /system/database/DB.php 文件中，根据系统的配置动态的定义了一个类 CI_DB，具体代码片段如下：

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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);