system/Session/Handlers/FileHandler.php
public function write($sessionID, $sessionData): bool
	{
		// If the two IDs don't match, we have a session_regenerate_id() call
		// and we need to close the old handle and open a new one
		if ($sessionID !== $this->sessionID && (! $this->close() || $this->read($sessionID) === false))
		{
			return false;
		}

		if (! is_resource($this->fileHandle))
		{
			return false;
		}
		elseif ($this->fingerprint === md5($sessionData))
		{
			return ($this->fileNew) ? true : touch($this->filePath . $sessionID);
		}

		if (! $this->fileNew)
		{
			ftruncate($this->fileHandle, 0);
			rewind($this->fileHandle);
		}

		if (($length = strlen($sessionData)) > 0)
		{
			for ($written = 0; $written < $length; $written += $result)
			{
				if (($result = fwrite($this->fileHandle, substr($sessionData, $written))) === false)
				{
					break;
				}
			}

			if (! is_int($result))
			{
				$this->fingerprint = md5(substr($sessionData, 0, $written));
				$this->logger->error('Session: Unable to write data.');

				return false;
			}
		}

		$this->fingerprint = md5($sessionData);

		return true;
	}
public function write($sessionID, $sessionData): bool
	{
		// If the two IDs don't match, we have a session_regenerate_id() call
		// and we need to close the old handle and open a new one
		if ($sessionID !== $this->sessionID && (! $this->close() || $this->read($sessionID) === false))
		{
			return false;
		}

		if (! is_resource($this->fileHandle))
		{
			return false;
		}
		elseif ($this->fingerprint === md5($sessionData))
		{
			return ($this->fileNew) ? true : touch($this->filePath . $sessionID);
		}

		if (! $this->fileNew)
		{
			ftruncate($this->fileHandle, 0);
			rewind($this->fileHandle);
		}

		if (($length = strlen($sessionData)) > 0)
		{
			for ($written = 0; $written < $length; $written += $result)
			{
				if (($result = fwrite($this->fileHandle, substr($sessionData, $written))) === false)
				{
					break;
				}
			}

			if (! is_int($result))
			{
				$this->fingerprint = md5(substr($sessionData, 0, $written));
				$this->logger->error('Session: Unable to write data.');

				return false;
			}
		}

		$this->fingerprint = md5($sessionData);

		return true;
	}
Write

Writes (create / update) session data

@param string $sessionID   Session ID
@param string $sessionData Serialized session data

@return boolean