Try this:
PHP Code: 
<form method="GET" action="enteryoururl.php">
Enter url: <input type="text" name="link" />
<input type="submit" value="Submit" />
</form>

<?php
if (isset($_GET["link"])) {
$url$_GET["link"];
echo 
'<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>'
;
}
?>
However, i would suggest you this one using the POST method:
PHP Code: 
<form method="POST" action="enteryoururl.php">
Enter url: <input type="text" name="link" />
<input type="submit" value="Submit" name="Submit" />
</form>

<?php
if (isset($_POST["link"]) and isset($_POST["Submit"])) {
$url$_POST["link"];
echo 
'<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>'
;
}
?>