<?php
 
// Define o intervalo da manhã:
$hora_manha_min = new DateTime("08:00");
$hora_manha_max = new DateTime("12:00");
 
// Define o intervalo da tarde:
$hora_tarde_min = new DateTime("13:00");
$hora_tarde_max = new DateTime("18:00");
 
// Intervalo de 15 minutos:
$intervalo = new DateInterval("PT15M");
 
// Lista de todos os horários:
$horarios = [];
 
// Adiciona na lista os horários da manhã com intervalo de 15 minutos:
$temp = clone $hora_manha_min;
 
while ($temp <= $hora_manha_max)
{
    $horarios[] = $temp;
    $temp = clone $temp;
    $temp->add($intervalo);
}
 
// Adiciona na lista os horários da tarde com intervalo de 15 minutos:
$temp = clone $hora_tarde_min;
 
while ($temp <= $hora_tarde_max)
{
    $horarios[] = $temp;
    $temp = clone $temp;
    $temp->add($intervalo);
}
 
// Horários ocupados:
$ocupado_1_ini = new DateTime("09:00");
$ocupado_2_ini = new DateTime("10:45");
 
// Remove da lista os horários ocupados:
if(($key = array_search($ocupado_1_ini, $horarios)) !== false) {
    echo "Removendo ", $horarios[$key]->format("H:i"), PHP_EOL;
    unset($horarios[$key]);
}
 
if(($key = array_search($ocupado_2_ini, $horarios)) !== false) {
    echo "Removendo ", $horarios[$key]->format("H:i"), PHP_EOL;
    unset($horarios[$key]);
}
 
// Imprime um horário aleatório:
echo $horarios[array_rand($horarios)]->format("H:i"), PHP_EOL;

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