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[RuntimeException] 
Could not start session because headers have already been sent. "/home/host1810523/radental.ru/htdocs/www/useful/index.php":2. (0)
/home/host1810523/dentsystem.ru/htdocs/www/bitrix/modules/main/lib/session/session.php:144
#0: Bitrix\Main\Session\Session->start()
	/home/host1810523/dentsystem.ru/htdocs/www/bitrix/modules/main/lib/session/kernelsessionproxy.php:47
#1: Bitrix\Main\Session\KernelSessionProxy->start()
	/home/host1810523/dentsystem.ru/htdocs/www/bitrix/modules/main/include.php:168
#2: require_once(string)
	/home/host1810523/dentsystem.ru/htdocs/www/bitrix/modules/main/include/prolog_before.php:19
#3: require_once(string)
	/home/host1810523/dentsystem.ru/htdocs/www/bitrix/modules/main/include/prolog.php:10
#4: require_once(string)
	/home/host1810523/dentsystem.ru/htdocs/www/bitrix/header.php:1
#5: require(string)
	/home/host1810523/radental.ru/htdocs/www/useful/index.php:2
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