Summary of php eval function usage

Definition and usage of eval

The eval() function evaluates the string as PHP code.

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

If the return statement is not called in the code string, NULL is returned. If there is a parsing error in the code, the eval() function returns false.

grammar
eval(phpcode)

Parameters to describe
phpcode required. Specifies the PHP code to calculate.

Hints and comments
Comment: the return statement immediately terminates the evaluation of the string.
Note: this function is useful for storing code in a database text field for later computation.
example

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

Output:
Copy the code as follows: This is a $string $time morning!
This is a beautiful winter morning!
The eval() function is also available in the CodeIgniter framework. In/system database/DB php file, according to the configuration of system dynamic CI_DB defines a class, specific code snippet below:?

 
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); 

This function replaces the value of a variable in a string and is usually used to process data from a database. The parameter code_str is the string to be processed. It is important to note that the string to be processed should conform to PHP's string format, with a semicolon at the end. The string processed using this function continues to the end of the PHP program.