Source

File: wp-admin/includes/class-wp-site-health.php

public function get_test_php_sessions() {
		$result = array(
			'label'       => __( 'No PHP sessions detected' ),
			'status'      => 'good',
			'badge'       => array(
				'label' => __( 'Performance' ),
				'color' => 'blue',
			),
			'description' => sprintf(
				'<p>%s</p>',
				sprintf(
					/* translators: 1: session_start(), 2: session_write_close() */
					__( 'PHP sessions created by a %1$s function call may interfere with REST API and loopback requests. An active session should be closed by %2$s before making any HTTP requests.' ),
					'<code>session_start()</code>',
					'<code>session_write_close()</code>'
				)
			),
			'test'        => 'php_sessions',
		);

		if ( function_exists( 'session_status' ) && PHP_SESSION_ACTIVE === session_status() ) {
			$result['status'] = 'critical';

			$result['label'] = __( 'An active PHP session was detected' );

			$result['description'] = sprintf(
				'<p>%s</p>',
				sprintf(
					/* translators: 1: session_start(), 2: session_write_close() */
					__( 'A PHP session was created by a %1$s function call. This interferes with REST API and loopback requests. The session should be closed by %2$s before making any HTTP requests.' ),
					'<code>session_start()</code>',
					'<code>session_write_close()</code>'
				)
			);
		}

		return $result;
	}