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 446Just 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}`;?>

If you want to avoid situations like the one described by @robert you may want to disable `shell_exec` and -as a consequence- the backtick operator.To do this just edit the `php.ini` file and add `shell_exec` to the `disable_functions` setting:    ; This directive allows you to disable certain functions.    ; It receives a comma-delimited list of function names.    ; https://php.net/disable-functions    disable_functions = "shell_exec"Then you can still use `exec()` to run terminal commands.