usort.php

<?php
$arreglo = [
    [
        "hora" => "07:00:00",
        "fecha" => "2019-07-30",
        "datetime" => "2019-07-30 07:00:00",
    ],
    [
        "hora" => "13:15:20",
        "fecha" => "2002-11-24",
        "datetime" => "2002-11-24 13:15:20",
    ],
    [
        "hora" => "15:27:32",
        "fecha" => "2007-10-29",
        "datetime" => "2007-10-29 15:27:32",
    ],
    [
        "hora" => "15:27:33",
        "fecha" => "2007-10-29",
        "datetime" => "2007-10-29 15:27:33",
    ],
];
echo "Arreglo antes de ordenar: ";
print_r($arreglo);

usort($arreglo, function ($a, $b) {
    return strcmp($a["datetime"], $b["datetime"]);
});
echo "Arreglo DESPUÉS de ordenar: ";
print_r($arreglo);

/*
Salida:
Arreglo antes de ordenar: Array
(
    [0] => Array
        (
            [hora] => 07:00:00
            [fecha] => 2019-07-30
            [datetime] => 2019-07-30 07:00:00
        )

    [1] => Array
        (
            [hora] => 13:15:20
            [fecha] => 2002-11-24
            [datetime] => 2002-11-24 13:15:20
        )

    [2] => Array
        (
            [hora] => 15:27:32
            [fecha] => 2007-10-29
            [datetime] => 2007-10-29 15:27:32
        )

    [3] => Array
        (
            [hora] => 15:27:33
            [fecha] => 2007-10-29
            [datetime] => 2007-10-29 15:27:33
        )

)
Arreglo DESPUÉS de ordenar: Array
(
    [0] => Array
        (
            [hora] => 13:15:20
            [fecha] => 2002-11-24
            [datetime] => 2002-11-24 13:15:20
        )

    [1] => Array
        (
            [hora] => 15:27:32
            [fecha] => 2007-10-29
            [datetime] => 2007-10-29 15:27:32
        )

    [2] => Array
        (
            [hora] => 15:27:33
            [fecha] => 2007-10-29
            [datetime] => 2007-10-29 15:27:33
        )

    [3] => Array
        (
            [hora] => 07:00:00
            [fecha] => 2019-07-30
            [datetime] => 2019-07-30 07:00:00
        )

)
*/