Fixing the “Cannot modify header information” PHP error

With every page load, the PHP error log shows:

PHP Warning: Cannot modify header information - headers already sent in /wp-content/plugins/simplr-registration-form/lib/wp-session-manager/includes/class-wp-session.php on line 128

To fix, change that line 128 from:

setcookie( WP_SESSION_COOKIE, $this->session_id . '||' . $this->expires . '||' . $this->exp_variant , $this->expires, COOKIEPATH, COOKIE_DOMAIN, $secure, $httponly );

to:

if ( ! headers_sent() ) {
	setcookie( WP_SESSION_COOKIE, $this->session_id . '||' . $this->expires . '||' . $this->exp_variant , $this->expires, COOKIEPATH, COOKIE_DOMAIN, $secure, $httponly );
}