【PHP】ある月の初日と末日を取得する方法（DateTimeクラス & Carbon編） ref: https://qiita.com/sola-msr/items/58479913ff62452b6bb6

file0.php

date_default_timezone_set('Asia/Tokyo');

$setYear = 2018;
$setMonth = 2;

$date = new DateTime();
$month['first_day'] = $date->setDate($setYear, $setMonth, 1)->format('Y-m-d'); // 日を1で固定値を入れている
$month['last_day']  = $date->setDate($setYear, $setMonth, 1)->format('Y-m-t'); // 日を1で固定値を入れている

var_dump($month);

/* 実行結果

array(2) {
  ["first_day"]=>
  string(10) "2018-02-01"
  ["last_day"]=>
  string(10) "2018-02-28"
}

*/

file1.php

$setYear = 2018;
$setMonth = 2;

$month['first_day'] = Carbon::create($setYear, $setMonth, 1)->firstOfMonth(); // 日を1で固定値を入れている
$month['last_day']  = Carbon::create($setYear, $setMonth, 1)->lastOfMonth();  // 日を1で固定値を入れている

var_dump($month);

/* 実行結果

array(2) {
  ["first_day"]=>
  string(10) "2018-02-01"
  ["last_day"]=>
  string(10) "2018-02-28"
}

*/

file2.txt

Psy Shell v0.9.6 (PHP 7.2.5-1+ubuntu18.04.1+deb.sury.org+1 — cli) by Justin Hileman
>>> use Carbon\Carbon;
>>> Carbon::create(2018, 1)->lastOfMonth();
=> Carbon\Carbon @1517356800 {#2846
     date: 2018-01-31 00:00:00.0 UTC (+00:00),
   }
>>> Carbon::create(2018, 2)->lastOfMonth();
=> Carbon\Carbon @1522454400 {#2839
     date: 2018-03-31 00:00:00.0 UTC (+00:00),
   }
>>> Carbon::create(2018, 3)->lastOfMonth();
=> Carbon\Carbon @1522454400 {#2844
     date: 2018-03-31 00:00:00.0 UTC (+00:00),
   }
>>> Carbon::create(2018, 6)->lastOfMonth();
=> Carbon\Carbon @1530316800 {#2847
     date: 2018-06-30 00:00:00.0 UTC (+00:00),
   }
>>> Carbon::create(2018, 2)->daysInMonth;
=> 31
>>>

file3.txt

Psy Shell v0.9.6 (PHP 7.2.5-1+ubuntu18.04.1+deb.sury.org+1 — cli) by Justin Hileman
>>> use Carbon\Carbon;
>>> Carbon::create(2018, 1)->lastOfMonth();
=> Carbon\Carbon @1517356800 {#2846
     date: 2018-01-31 00:00:00.0 UTC (+00:00),
   }
>>> Carbon::create(2018, 2)->lastOfMonth();
=> Carbon\Carbon @1519776000 {#2839
     date: 2018-02-28 00:00:00.0 UTC (+00:00),
   }
>>> Carbon::create(2018, 3)->lastOfMonth();
=> Carbon\Carbon @1522454400 {#2844
     date: 2018-03-31 00:00:00.0 UTC (+00:00),
   }
>>> Carbon::create(2018, 6)->lastOfMonth();
=> Carbon\Carbon @1530316800 {#2847
     date: 2018-06-30 00:00:00.0 UTC (+00:00),
   }
>>> Carbon::create(2018, 2)->daysInMonth;
=> 28
>>>

file4.txt

2018-02-28 + 1 = 2018-03-01