Javascript 使用目标作为动态创建的php页面的iframe

我有一个动态php脚本，我希望在iframe中显示它的响应

<div id="plotpnl">
    <b>Graph :</b><br><br>
    <form action="create_charts.php" method="post" target="iframe_a">
        <font color="#587E91"><b>Strats :  </b></font>
        <input type="text" style="width:220px; margin:0 auto;" name="stratlist" placeholder="comma separated strat list"/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;

        <font color="#587E91"><b>Columns :  </b></font>
        <input type="text" style="width:265px; margin:0 auto;" name="collist" placeholder="comma separated columns list" />&nbsp;&nbsp;

        <input type="submit" value="Plot" style="color:#587E91;font-weight:bold"/> &nbsp; <input type="button" value="Wipe" style="color:#587E91;font-weight:bold" onClick="this.form.reset()"/>
        <iframe width="100%" height="300px" src="DUMMY.php" name="iframe_a"></iframe>
    </form>
</div>

当我使用target=iframe\u a时，我无法理解为什么内容没有重定向到iframe。如果我使用target=“\u blank”，代码显示良好

$stratlist=$_POST['stratlist']; 
$collist=$_POST['collist']; 

#echo "stratlist : $stratlist , collist : $collist , inst_list : $instlist"; 

$cmd1 = "makeaphpcode.sh \"$stratlist\" \"$collist\" \"pnl\""; 
exec($cmd1); 


$filename="plot.pnl.$stratlist.$collist.php"; 
$filename=str_replace(',','.',$filename); 
$filename=str_replace(' ','',$filename); 


if (!headers_sent($file, $line)){ 
   header("Location:mysite.com/$filename"); 
   exit; 
}else{ 
   echo "Headers sent in $file on line $line"; 
   exit;