<!-- Collection of code snippets by Arne Vajhøj -->
<!-- posted to eksperten.dk, usenet and other places (2002-now) -->
<?php
/*

CREATE TABLE iddtval (
    id INTEGER NOT NULL,
    dt DATE NOT NULL,
    val CHAR(1),
    PRIMARY KEY(id)
);
INSERT INTO iddtval VALUES(1,'2019-09-01','A');
INSERT INTO iddtval VALUES(2,'2019-09-03','A');
INSERT INTO iddtval VALUES(3,'2019-09-04','A');
INSERT INTO iddtval VALUES(4,'2019-09-01','B');
INSERT INTO iddtval VALUES(5,'2019-09-06','B');
INSERT INTO iddtval VALUES(6,'2019-09-09','A');
INSERT INTO iddtval VALUES(7,'2019-09-01','C');

*/

class Item {
    public $dt;
    public $vals;
    public function __construct($dt, $vals) {
        $this->dt = $dt;
        $this->vals = $vals;
    }
}

function load() {
    $res = array();
    $con = new mysqli('localhost', 'root', '', 'Test');
    $stmt = $con->prepare("SELECT dt, GROUP_CONCAT(val) AS vals FROM iddtval GROUP BY dt");
    $stmt->execute();
    $rs = $stmt->get_result();
    while($row = $rs->fetch_array(MYSQLI_ASSOC)) {
        $dt = $row['dt'];
        $vals = $row['vals'];
        $res[] = new Item($dt, $vals);
    }
    $stmt->close();
    $con->close();
    return $res;
}

function display($data) {
    foreach($data as $item) {
        $dt = $item->dt;
        $vals = $item->vals;
        echo "$dt $vals\r\n";
    }
}

$data = load();
display($data);
?>