这段代码为啥不能输出值？求教大神

<FORM method="post" action="" target=_blank>
<TABLE border=0 cellSpacing=0 cellPadding=0 width=200>
<TBODY>
<TR>
<TD id=term_search_l></TD>
<TD class=term_search_m o>
  <p>
    <INPUT type="text" value="请输入材料名称" name="materialname" style="font-family:'微软雅黑' '宋体';"  onfocus="if (value==defaultValue) value=''; " onblur="if(!value) {value=defaultValue}">
  </p></TD>
</TR>                                                                                           
</TBODY>
</TABLE>
</FORM>

<div id="button1">
<input type="submit" name="submit" id="button" value="" style="color:#FFF; font-family:"微软雅黑"; font-size:24px"; onmousedown="this.blur();this.style.backgroundPosition='left -29px'" onmouseup="this.blur();this.style.backgroundPosition='left top'"/>
</div>

<p><?php
if($_POST[submit]=="")
{
    echo $_POST[materialname];    
}
?></p>
</div>