执行运算符

PHP 支持一个执行运算符：反引号（``）。注意这不是单引号！PHP 将尝试将反引号中的内容作为 shell 命令来执行，并将其输出信息返回（即，可以赋给一个变量而不是简单地丢弃到标准输出）。使用反引号运算符“`”的效果与函数shell_exec()相同。

<?php
$output = `ls -al`;
echo "<pre>$output</pre>";
?>

Note:

反引号运算符在激活了安全模式或者关闭了shell_exec()时是无效的。

Note:

与其它某些语言不同，反引号不能在双引号字符串中使用。

参见手册中程序执行函数，popen()，proc_open()以及PHP 的命令行模式。

Just a general usage note. I had a very difficult time solving a problem with my script, when I accidentally put one of these backticks at the beginning of a line, like so:
[lots of code]
`  $URL = "blah...";
[more code]
Since the backtick is right above the tab key, I probably just fat-fingered it while indenting the code.
What made this so hard to find, was that PHP reported a parse error about 50 or so lines *below* the line containing the backtick. (There were no other backticks anywhere in my code.) And the error message was rather cryptic:
Parse error: parse error, expecting `T_STRING' or `T_VARIABLE' or `T_NUM_STRING' in /blah.php on line 446
Just something to file away in case you're pulling your hair out trying to find an error that "isn't there."

You can use variables within a pair of backticks (``).
<?php
  $host = 'www.wuxiancheng.cn';
  echo `ping -n 3 {$host}`;
?>