Code: 
<form method="get" action="test.php">
Enter url: <input type="text" name="link" />
<input type="submit" value="Submit" />
</form>

<?php
$url= $_GET["link"];
?>

<object id="flashplayer" classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" width="600" height="400">
<param name="movie" value="player.swf" />
<param name="allowFullScreen" value="true" />
<param name="allowScriptAccess" value="always" />
<param name="FlashVars" value="plugins=plugins/proxy.swf&proxy.link=$url" />
<embed name="flashplayer" src="player.swf"  FlashVars="plugins=plugins/proxy.swf&proxy.link=$url"  type="application/x-shockwave-flash" allowfullscreen="true"  allowScriptAccess="always" width="600" height="400" />
</object>
Modified it according to my knowledge. Do test it

Method should be GET in form when you are using GET, and the syntax of get is $_GET