PHP致命错误：在heroku

Question

我正在尝试将Heroku上托管的PHP应用程序连接到位于其他位置的Microsoft SQL Server数据库。我已经写出了连接代码，但每当我将它推送到Heroku时，Heroku只会返回PHP Fatal error: Call to undefined function sqlsrv_connect() in /app/connection.php on line 5。任何人都可以帮助我吗？

connection.php：

<?php

$serverName = "nameOfHost.net\serverInstance, 1234"; 
$connectionInfo = array( "Database"=>"competition", "UID"=>"username", "PWD"=>"password");
$conn = sqlsrv_connect( $serverName, $connectionInfo);

if($conn) {
    echo "Connection established. <br />";
} else {
    echo "Connection cannot be established. <br />";
    die( print_r( sqlsrv_errors(), true));
}

?>

谢谢！