<?php
function get_ciclo($dia_inicial=21, $dia_final=20, $data_referencia) {
    $mes = date('m', $data_referencia) - 1; // começa no mês anterior
    $ano = date('Y', $data_referencia);
    if (date("d", $data_referencia) < $dia_inicial) {
        $mes--; // // nesse caso, volta mais um mês
    }
    return array(
        date("Y-m-d", mktime(0, 0, 0, $mes, $dia_inicial, $ano)),
        date("Y-m-d", mktime(0, 0, 0, $mes + 1, $dia_final, $ano))
    );
}
 
$dia_inicial = 21;
$dia_final = 20;
$mes = 10;
$ano = 2021;
while (true) {
    // gera o ciclo do mês atual
    print_r(get_ciclo($dia_inicial, $dia_final, mktime(0, 0, 0, $mes, $dia_inicial, $ano)));
 
    // avança um mês
    $data = mktime(0, 0, 0, $mes + 1, $dia_inicial, $ano);
    $mes = date('m', $data);
    $ano = date('Y', $data);
    if ($ano == 2022 && $mes == 10) // 09/2022 é o último mês, se chegou em 10/2022, encerra
        break;
}
 

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