php表单提交数据到另一页面，显示空白怎么办

<form action="post_data.php" ,method="post">
数据一：<input type="text" name="akak">
<br>
数据二：<input type="text" name="zkzk">
<br>
<input type="submit" name="result" value="提交">
 
 
</form>

 <!--?php
 if(!empty($_post)){
 $a1 = $_post[&#39;akak&#39;];
  $a2 = $_post[&#39;zkzk&#39;];
echo " <hr /-->";
 
echo "z1= $a1,z2= $a2";
echo " <hr>";
var_dump($_post);}
else{
echo "页面没有值传过来！！";
}

<form action="post_data.php" method="post">
数据一：<input type="text" name="akak">
<br>
数据二：<input type="text" name="zkzk">
<br>
<input type="submit" name="result" value="提交">
 
 
</form>

<!--?php
 if(!empty($_POST)){
 $a1 = $_POST[&#39;akak&#39;];
  $a2 = $_POST[&#39;zkzk&#39;];
echo " <hr /-->";
 
echo "z1= $a1,z2= $a2";
echo " <hr>";
var_dump($_POST);}
else{
echo "页面没有值传过来！！";
}
   ?>