I go back and forth between ColdFusion and PHP quite a bit. One of the things I love about ColdFusion is the Query of query -- the ability to query the database, then drill down further in your recordset for a subset of the original query. This has many uses, and is very fast because it does not make a round trip to the database. Recently when building a PHP site, I had a few instances where query of query would have been really nice. I decided to write a function to perform a simple filter on a recordset. It is not a "query" of an existing recordset, but it is the next best thing. It can match records on one field if they exist in a comma-separated list. It can also be used to return a list of distinct records:

function queryOfQuery($rs, // The recordset to query
  $fields = "*", // optional comma-separated list of fields to return, or * for all fields
  $distinct = false, // optional true for distinct records
  $fieldToMatch = null, // optional database field name to match
  $valueToMatch = null) { // optional value to match in the field, as a comma-separated list

  $newRs = Array();
  $row = Array();
  $valueToMatch = explode(",",$valueToMatch);
  $matched = true;
  mysql_data_seek($rs, 0);
  if($rs) {
    while ($row_rs = mysql_fetch_assoc($rs)){
      if($fields == "*") {
        if($fieldToMatch != null) {
          $matched = false;
          if(is_integer(array_search($row_rs[$fieldToMatch],$valueToMatch))){
            $matched = true;
          }
        }
        if($matched) $row = $row_rs;
      }else{
        $fieldsArray=explode(",",$fields);
        foreach($fields as $field) {
          if($fieldToMatch != null) {
            $matched = false;
            if(is_integer(array_search($row_rs[$fieldToMatch],$valueToMatch))){
              $matched = true;
            }
          }
          if($matched) $row[$field] = $row_rs[$field];
        }
      }
      if($matched)array_push($newRs, $row);
    };
    if($distinct) {
      sort($newRs);
      for($i = count($newRs)-1; $i > 0; $i--) {
        if($newRs[$i] == $newRs[$i-1]) unset($newRs[$i]);
      }
    }
  }
  mysql_data_seek($rs, 0);
  return $newRs;
}

Sample uses:

Create the initial recordset:

<?php
mysql_select_db($database_northwind, $northwind);
$query_rs = "SELECT ProductID, ProductName, UnitPrice, UnitsInStock FROM Products";
$rs = mysql_query($query_rs, $northwind) or die(mysql_error());
?>

Then use the function in one of the following ways:

// Return all records from rs where ProductID is one of the list
$blah = queryOfQuery($rs, "*", false, "ProductID", "3,4,6,10,12");

// Return ProductName field from rs where productid is one of the list
$blah = queryOfQuery($rs, "ProductName", false, "ProductID", "3,4,6,10,12");

// Return distinct UnitsInStock counts from rs
$blah = queryOfQuery($rs, "UnitsInStock", true);

After returning the array, you can loop through it:

<table border="1">
  <tr>
    <td>ProductID</td>
    <td>ProductName</td>
    <td>UnitPrice</td>
    <td>UnitsInStock</td>
  </tr>
<?php foreach($blah as $row_rs) { ?>
  <tr>
    <td><?php echo $row_rs['ProductID']; ?></td>
    <td><?php echo $row_rs['ProductName']; ?></td>
    <td><?php echo $row_rs['UnitPrice']; ?></td>
    <td><?php echo $row_rs['UnitsInStock']; ?></td>
  </tr>
<?php } ?>
</table>

The Northwind sample database for MySQL that I use in examples can be downloaded from here. It is based on the MS SQL Server version of the database.

Download a sample PHP page using the function here.