How do I get the first day of the week for the current locale? (PHP / L8N)

From CLDR's supplemental data (release 36, October 2019):

<firstDay day="mon" territories="001 AD AI AL AM AN AR AT AX AZ BA BE BG BM BN BY CH CL CM CR CY CZ DE DK EC EE ES FI FJ FO FR GB GE GF GP GR HR HU IE IS IT KG KZ LB LI LK LT LU LV MC MD ME MK MN MQ MY NL NO NZ PL RE RO RS RU SE SI SK SM TJ TM TR UA UY UZ VA VN XK"/>
<firstDay day="fri" territories="MV"/>
<firstDay day="sat" territories="AE AF BH DJ DZ EG IQ IR JO KW LY OM QA SD SY"/>
<firstDay day="sun" territories="AG AS AU BD BR BS BT BW BZ CA CN CO DM DO ET GT GU HK HN ID IL IN JM JP KE KH KR LA MH MM MO MT MX MZ NI NP PA PE PH PK PR PT PY SA SG SV TH TT TW UM US VE VI WS YE ZA ZW"/>

<firstDay day="sun" territories="GB" alt="variant" references="Shorter Oxford Dictionary (5th edition, 2002)"/>

(territory 001 is the UN M49 area code for "World")

Run this command in the shell (bash or sh or etc):

locale first_weekday

In PHP, call a process to run this command in sh, and get it's output. In Python, I does a thing like this:

def getLocaleFirstWeekDay():
  return int(os.popen('locale first_weekday').read())-1