heroku上的php路径在exec中不起作用

我在Heroku上得到了一些PHP应用程序。当我启用PHP时，我也运行这个命令，我在Stackoverflow中找到了这个命令：

heroku config:add LD_LIBRARY_PATH=/app/php/ext:/app/apache/lib
Setting config vars and restarting zfq... done, v7
LD_LIBRARY_PATH: /app/php/ext:/app/apache/lib

命令行中的此命令运行良好：

heroku run ./php/bin/php -v
Running `./php/bin/php -v` attached to terminal... up, run.3208
PHP 5.3.10 (cli) (built: Mar 26 2012 08:15:52)
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies

但是这个代码：

$nickname = $_POST['nickname'];
$filename = "1.txt";
$cmd      = './php/bin/php -v > "' . $filename . '" 2>&1 &';
exec($cmd, $output);
$contents = file_get_contents($filename);
unlink($filename);
exit_ajax($contents);

返回sh: ./php/bin/php: not found

如果我更改/php/bin/php到~/php/bin/php-返回false

致/php-返回sh: ./php: not found