<!-- Collection of code snippets by Arne Vajhøj -->
<!-- posted to eksperten.dk, usenet and other places (2002-now) -->
<?php
/*
CREATE TABLE grp (id INTEGER NOT NULL, person_id INTEGER NOT NULL, PRIMARY KEY(id));
INSERT INTO grp VALUES(1, 1005);
INSERT INTO grp VALUES(2, 1021);
INSERT INTO grp VALUES(3, 1002);
INSERT INTO grp VALUES(4, 1018);
INSERT INTO grp VALUES(5, 1015);
INSERT INTO grp VALUES(6, 1001);
INSERT INTO grp VALUES(7, 1022);
INSERT INTO grp VALUES(8, 1019);
INSERT INTO grp VALUES(9, 1006);
INSERT INTO grp VALUES(10, 1026);
INSERT INTO grp VALUES(11, 1014);
INSERT INTO grp VALUES(12, 1029);
INSERT INTO grp VALUES(13, 1003);
INSERT INTO grp VALUES(14, 1025);
INSERT INTO grp VALUES(15, 1017);
INSERT INTO grp VALUES(16, 1007);
INSERT INTO grp VALUES(17, 1028);
INSERT INTO grp VALUES(18, 1011);
INSERT INTO grp VALUES(19, 1027);
INSERT INTO grp VALUES(20, 1002);
INSERT INTO grp VALUES(21, 1012);
INSERT INTO grp VALUES(22, 1012);
INSERT INTO grp VALUES(23, 1027);
INSERT INTO grp VALUES(24, 1005);
INSERT INTO grp VALUES(25, 1016);
INSERT INTO grp VALUES(26, 1010);
INSERT INTO grp VALUES(27, 1024);
INSERT INTO grp VALUES(28, 1004);
INSERT INTO grp VALUES(29, 1016);
INSERT INTO grp VALUES(30, 1013);
INSERT INTO grp VALUES(31, 1009);
INSERT INTO grp VALUES(32, 1008);
INSERT INTO grp VALUES(33, 1023);
INSERT INTO grp VALUES(34, 1020);
SELECT * FROM grp;
SELECT COUNT(*) AS total FROM grp;
SELECT person_id,COUNT(*) AS n FROM grp GROUP BY person_id;
DROP TABLE grp;
 */

function loadraw() {
    $con = new mysqli('localhost', 'root', '', 'Test');
    $stmt = $con->prepare('SELECT id,person_id FROM grp ORDER BY RAND()');
    $stmt->execute();
    $res = $stmt->get_result();
    $raw = $res->fetch_all(MYSQLI_ASSOC);
    $res->free();
    $stmt->close();
    $con->close();
    return $raw;
}

function init(&$onegroup, &$ingroup, &$dupl) {
    $onegroup = array();
    $ingroup = array();
    $dupl = array();
}

function process_row($row, &$ingroup, &$onegroup, &$dupl, &$groups, $max) {
    if(array_key_exists($row['person_id'], $ingroup)) {
        $dupl[] = $row;
    } else {
        $onegroup[] = $row;
        $ingroup[$row['person_id']] = true;
    }
    if(count($onegroup) == $max) {
        $groups[] = $onegroup;
        $olddupl = $dupl;
        init($onegroup, $ingroup, $dupl);
        foreach($olddupl as $row) {
            process_row($row, $ingroup, $onegroup, $dupl, $groups, $max);
        }
        if(count($dupl) > 0) throw new Exception("Could not complete grouping");
    }
}

function dogrouping($raw, $max)
{
    $groups = array();
    init($onegroup, $ingroup, $dupl);
    foreach($raw as $row) {
        process_row($row, $ingroup, $onegroup, $dupl, $groups, $max);
    }
    $olddupl = $dupl;
    $dupl = array();
    foreach($olddupl as $row) {
        process_row($row, $ingroup, $onegroup, $dupl, $groups, $max);
    }
    if(count($dupl) > 0) throw new Exception("Could not complete grouping");
    if(count($onegroup) > 0) {
        $groups[] = $onegroup;
    }
    return $groups;
}

$raw = loadraw();
echo "Raw:\r\n";
foreach($raw as $row) {
    echo $row['id'] . ' ' . $row['person_id'] . "\r\n";
}

echo "\r\n";

$grouped = dogrouping($raw, 10);
$gn = 1;
foreach($grouped as $group) {
    echo "Group $gn:\r\n";
    foreach($group as $row) {
        echo '  ' . $row['id'] . ' ' . $row['person_id'] . "\r\n";
    }
    $gn++;
}

?>