I have been struggling with it for a week...I want to use Ajax in datatables to pass some parameters to my PHP file for server side searching. But it seems my PHP file got nothing. But the datatable works just fine and the table can return from sever side processing and display on the html page correctly. However, var_dump( $_POST['species']) returns null.

  $(document).ready(function() {
    console.log(species);
    $("#table").DataTable({
      "destroy": true,
      "serverSide": true,
      "processing": true,
      "paging": false,
      "searching": false,

      columns: [{
          data: "dRid"
        },
        {
          data: "chromosome"
        },
        {
          data: "position"
        }
      ],
      ajax: {
        type: "POST",
        data: {
          "species": species
        },
        url: "./php/search_test.php"
      }

    });
  });

<div class="container text-center" id="tablecontainer" name="tablecontainer">
  <table id="table" class="display nowrap row-border strip p-2" cellspacing="0" width="100%">

    <thead class="thead-dark">
      <tr>
        <th>
          <center>directRMDB ID</center>
        </th>
        <th>
          <center>Chromosome</center>
        </th>
        <th>
          <center>Position</center>
        </th>

      </tr>
    </thead>
  </table>
</div>

'''php

ini_set ('memory_limit',  '2048M');
$sql_details = array(
'host' => 'localhost',
'user' => 'root',
'pass' => '',
'db'   => 'xxxxx'
);

// DB table to use
$table = 'homo_sapiens';

//if I try this here: $species=$_POST["species"]; echo $species;It //returns: Warning: Undefined array key "species".

// Table's primary key
$primaryKey = 'dRid';

// column
$columns = array(
array( 'db' => 'dRid', 'dt' => 'dRid' ),
array( 'db' => 'chr', 'dt' => 'chromosome' ),
array( 'db' => 'pos',  'dt' => 'position' ),


);

// Include SQL query processing class
require( 'ssp.class.php' );

echo json_encode(
SSP::complex( $_POST, $sql_details, $table, $primaryKey,$columns)
);

Could someone help me with it? Thanks.