PHP：无法使用shell_exec（）启动服务器

Question

I want to create my own little control panel for my minecraft server,

I got problems with executing commands in php.

So if I want to execute a .sh file in PHP It works, I tried to do an output with

echo "xyz"

This works, but if I try to start a server, it doesnt..

My Server-Start-File:

echo "$1 's Server is starting with $2 MB of RAM"
screen -AmdS $1 java -Xmx$2M -jar spigot1649.jar

My PHP-Script:

<?php
$x = shell_exec('./start.sh test 512');
if($x) {
echo "Started";
echo "" . shell_exec('./start.sh test 512');
} else {
echo "Failed";
}
?>