Home Backend Development PHP Tutorial A brief discussion on the definition and usage of PHP eval() function

A brief discussion on the definition and usage of PHP eval() function

Dec 30, 2016 am 10:27 AM

eval() 函数把字符串按照 PHP 代码来计算。

该字符串必须是合法的 PHP 代码,且必须以分号结尾。

如果没有在代码字符串中调用 return 语句,则返回 NULL。如果代码中存在解析错误,则 eval() 函数返回 false。

语法

eval(phpcode)
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浅谈PHP eval()函数定义和用法

提示和注释

注释:返回语句会立即终止对字符串的计算。

注释:该函数对于在数据库文本字段中供日后计算而进行的代码存储很有用。

例子

<?php
$string = "beautiful";
$time = "winter";
  
$str = &#39;This is a $string $time morning!&#39;;
echo $str. "<br />";
  
eval("\$str = \"$str\";");
echo $str;
?>  
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输出:

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

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:

if ( ! isset($active_record) OR $active_record == TRUE)
 {
 require_once(BASEPATH.&#39;database/DB_active_rec.php&#39;);
  
 if ( ! class_exists(&#39;CI_DB&#39;))
 {
  eval(&#39;class CI_DB extends CI_DB_active_record { }&#39;);
 }
 }
 else
 {
 if ( ! class_exists(&#39;CI_DB&#39;))
 {
  eval(&#39;class CI_DB extends CI_DB_driver { }&#39;);
 }
 }
  
 require_once(BASEPATH.&#39;database/drivers/&#39;.$params[&#39;dbdriver&#39;].&#39;/&#39;.$params[&#39;dbdriver&#39;].&#39;_driver.php&#39;);
  
 // Instantiate the DB adapter
 $driver = &#39;CI_DB_&#39;.$params[&#39;dbdriver&#39;].&#39;_driver&#39;;
 $DB = new $driver($params);
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