How to check if array sent via post in php

Question

This is my html page --

<select name="search_by_manager[]" data-placeholder="Search by Manager" multiple class="select2">
<?php
$result = $linkID1->query("select user_id,fullname from user_details order by user_id asc")
or
die("error");

while($row = mysqli_fetch_array($result))
{
    echo "<option value='$row[user_id]'>$row[fullname]</option>";
}
 ?>
</select>

This is my php page where data is posted --

if(!empty($_POST["search_by_manager"]))
{
    $search_by_manager = $_POST["search_by_manager"];
    $flag2 = 1;

    $limit = count($search_by_manager);

    for($i=0;$i<$limit;$i++)
    {
        $search_by_manager[$i] = mysql_real_escape_string($search_by_manager[$i]);
        $qry_manager = "'";
        $qry_manager .= "$search_by_manager[$i]";
        $qry_manager .= "',";
    }
    $qry_manager = substr($qry_manager,0,-1);
}

Here the problem is that, its not getting inside the if block, how to check whether values sent via post or not. Normal data can be easily checked using isset() function but how to check whether post array is empty or not.