允许的内存大小已用完134217728字节（尝试分配16384字节）

Question

我是php的新手，它试图通过数据库（mysql）获取导航栏的数据，但显示如下：

  

允许的内存大小为134217728字节，已用尽（试图分配16384字节

<?php
$con=mysqli_connect("localhost","root","","menu");
// Check connection
if (mysqli_connect_errno())
{
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

// Perform queries 

function get_menu_tree($id) 
{
    global $con;
    $menu = "";
    $sqlquery = " SELECT * FROM mymenu";
    $res=mysqli_query($con,$sqlquery);
    while($row=mysqli_fetch_array($res,MYSQLI_ASSOC)) 
    {
           $menu ="<li><a href='".$row['link']."'>".$row['title']."</a>";

           $menu = "<ul>".get_menu_tree($row['id'])."</ul>"; //call  recursively

           $menu = "</li>";

    }

    return $menu;
} 
?>
<h1>Create Nested menu Tree by Mysql php</h1>
<ul class="main-navigation">
<?php echo get_menu_tree(1);//start from root menus having id 0 ?>
</ul>