Summary of PHP eval function usage

Definitions and usage
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
? eval (Phpcode)
　　

Parameter description
Phpcode required. 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
?

The code is as follows	Copy Code
<?php $string = "Beautiful"; $time = "Winter"; $str = ' is a $string $time morning! '; echo $str. "<br/>"; Eval ("$str =" $str ";"); Echo $str; ?>

Output:

The code is as follows	Copy Code
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:?

The code is as follows

Copy Code

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 can be used to substitute the value of a variable in a string, usually on data that is processed on the database. Parameter code_str is the string to be processed. It is worth noting that the string to be processed conforms to the PHP string format, with a semicolon at the end. The string that is processed using this function is followed by the end of the PHP program.