ok , questo e'il mio problema

Ho un campo file all'interno di una form

ammettiamo che il campo non viene riempito e non viene inviato nessun file, posso io impostare nel file php al quale viene invitao il form un controllo tale che mi inserisce una immagine di default?

Mi spiego meglio, il campo file si chiama "userfile" e nel php vorrei inserire una riga di questo genere, la variabile $add e' quella che funzione nello script :


codice:

<body bgcolor=white>

<basefont size=2 face=arial>

Add Article


<?

        include ("template.inc");
        include ("config.php");


if ($userfile_size >500000){$msg=$msg."Your uploaded file size is more than 500KB so please reduce the 

file size and then upload. Visit the help page to know how to reduce the file size.
";
$file_upload="false";}

if (!($userfile_type =="image/pjpeg" OR $userfile_type=="image/gif" OR 

$userfile_type=="image/png")){$msg=$msg."Your uploaded file must be of JPG or GIF. Other file types are 

not allowed
";
$file_upload="false";}




if  ($userfile = "") { $add = "empty.jpg"; } else { $add = "$userfile_name"; } 
// the path with the file name where the file will be stored, upload is the directory name. 



if(move_uploaded_file ($userfile, $add)){
// do your coding here to give a thanks message or any other thing.
}else{echo "";}

	$summary_template = "t_summary.html";
	$article_template = "t_article.html";
	$max_summary = 5;

	function summary_page ($subject, $date, $summary, $article_id, $add)
	{
		global $summary_template;
        	$t = new Template();
        	$t->set_file("SummaryPage", $summary_template);
		$article_url = "article_".$article_id.".html";
		$date = nl2br($date);
		$summary =  nl2br($summary);
		$userfile = $add;
		$t->set_var( array(
				"subject" => $subject,
				"date"    => $date,
				"summary" => $summary,
				"userfile" => $userfile,
				"article_url" => $article_url
				));
		$t->parse("Summary", "SummaryPage");
		return $t->get_var("Summary");
	}

	function main_page ($subject, $date, $summary, $article_id, $body, $add)
	{
		global $article_template;

                $t = new Template();
                $t->set_file("ArticlePage", $article_template);
                $article_url = "article_".$article_id.".html";
                $date = nl2br($date);
                $summary =  nl2br($summary);
                $body =  nl2br($body);
		$userfile = $add;
		$t->set_var( array(
                                "subject" => $subject,
                                "date"    => $date,
                                "summary" => $summary,
                                "body" => $body,
				"userfile" => $userfile,
				"article_url" => $article_url
                                ));
                $t->parse("Article", "ArticlePage");
                return $t->get_var("Article"); 
	}

	function add_article($filename, $news)
	{

		if(file_exists($filename)){
			$fh = fopen($filename, "r");
			$old_news = fread($fh, filesize($filename));
			fclose($fh); 
		}

		/* TODO: Multipage articles
			preg_match_all("", $old_news, $matches;
		
			if( count($matches[0]) >= $max_summary){
				$oldfilename = $filename.($matches[0][0]+1);
			} 
		*/

		$fh = fopen($filename, "w");
		$news = stripslashes($news);
		fwrite($fh, "\n\n$news $old_news");
		fclose($fh);
	}


		if(strcmp($subject, "")){
		if(!(strcmp($passwd, $password))){	
			add_article("article_summary.html", summary_page($subject, $date, $summary, 

$article_id, $add));
			add_article("article_$article_id.html", main_page($subject, $date, $summary, 

$article_id, $body, $add));
			echo "

 Article has been added! 

";
		}else{
			echo "

 Password is wrong! ";
		}
	}
?>




<form enctype=multipart/form-data action=news.php name=postnews method=post onSubmit="return 

(confirm('Are you sure you wanna send the message?'));">
<table border=0>
<tr> <td> Password: </td><td> <input type=password name=passwd size=30> </td></tr>
<tr> <td> Re-type Password: </td><td> <input type=password name=passwd2 size=30> </td></tr>
<tr> <td> Subject: </td><td> <input type=text name=subject size=30> </td></tr>
<tr> <td> Article ID: </td><td> <input type=text name=article_id value=<? echo date("d_m_Y__H-i-s"); ?> 

size=30> </td></tr>
<tr> <td> Date: </td><td> <input type=text name=date value=<? echo date("d/m/Y\n"); ?> size=30 

></td></tr>
<tr> <td> Summary: </td><td> <textarea name=summary 

onKeyDown="countAreaChars(this.form.feedback,this.form.leftChars,150);"
onKeyUp="countAreaChars(this.form.summary,this.form.leftChars,150);" rows=5 cols=30 

wrap=soft></textarea>



<input type=text name=leftChars readonly size=3 maxlength=3 value="150"> Letters Remaining


 </td></tr>
<tr> <td> Body: </td><td> <textarea name=body rows=15 cols=30></textarea> </td></tr>

<tr> <td> Picture: </td><td><input type=file name=userfile size=35 onChange="estensione(this.form, 

this.value)"></td></tr>
<tr> <td> Remember! All Pictures Uploaded
MUST be JPG or GIF
and have dimensions
width: 450 px 

height: 225 px  </td><td></td></tr> 
</table>
<input type=submit name=submit onclick="return controllo();" class="smallboldtext" value="Изпратете">
<input type="reset" value="Откажете" name="Откажете" class="smallboldtext">
</form>

tutto sta nella riga di comando

if ($userfile = "") { $add = "empty.jpg"; } else { $add = "$userfile_name"; }

che ho evidenziato in rosso, NON FUNZIONA AL MOMENTO, ovvero se non inserisco un-immagine il file php mi restituisce il campo vuoto, come fare?