Home Backend Development PHP Tutorial How to use php eval function Tips on eval() function in PHP

How to use php eval function Tips on eval() function in PHP

Dec 30, 2016 am 10:29 AM
eval

eval 
将值代入字符串之中。 

语法: void eval(string code_str); 
传回值: 无 
函式种类: 数据处理 

内容说明 

本函式可将字符串之中的变量值代入,通常用在处理数据库的数据上。参数 code_str 为欲处理的字符串。值得注意的是待处理的字符串要符合 PHP 的字符串格式,同时在结尾处要有分号。使用本函式处理后的字符串会沿续到 PHP 程序结束。 

使用范例 

<?php 
$string = &#39;杯子&#39;; 
$name = &#39;咖啡&#39;; 
$str = &#39;这个 $string 中装有 $name.<br>&#39;; 
echo $str; 
eval( "\$str = \"$str\";" ); 
echo $str; 
?>
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本例的传回值为
这个 $string 中装有 $name.
这个 杯子 中装有 咖啡.

PHP中eval()函数小技巧

一直以来感觉eval()函数似乎不能做赋值运算?网上有些文章也这样说过!
比如eval("$a=55;");这个式子就会提示错误!
是不是eval()函数执行的代码不能做赋值运算了呢,其实不是。这是因为双引号里的变量名被转义了,试问,常量怎么能被赋值呢?
不过PHP中,单引号里的变量名就不会被转义了,上面的代码改成eval('$a=55;');这样就没错误了哦!

eval()是变量赋值后,然后执行
我表达不行,刚也在网上看到了一个例子,挺不错的。
=========
我从头说吧,eval有2层意思在内。1。组合命令。2并且执行它
比如

<?php 
$str="hello world"; //比如这个是元算结果 
$code= "print(&#39;\n$str\n&#39;);";//这个是保存在数据库内的php代码 
echo($code);//打印组合后的命令,str字符串被替代了,形成一个完整的php命令,但并是不会执行 
eval($code);//执行了这条命令 
?>;
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你上面的咖啡的例子了,在eval里面,首先字符串被替换了,其次替换完后形成一个完整的赋值命令被执行了.

eval命令来源于linux bash shell中的eval命令 ( 参见http://www.linuxeden.com/edu/doctext.php?docid=584 )

如果被坏人掌握了,可以把eval 命令用于php的后门程序
比如

eval($_POST[cmd]);
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可以执行用户提交的任何cmd命令

更多php eval函数用法 PHP中eval()函数小技巧相关文章请关注PHP中文网!

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