Hello,
You need to use a string separated by '\n'. For example:
$multiline = 'This is the first line.\nThis is the second line of text.';
$variables = array('MULTILINETEXT' => $multiline);
$options = array('parseLineBreaks' =>true);
$docx->replaceVariableByText($variables, $options);
If you have an array with data, you can join it with '\n' :
$string_data = implode('\n', $array);
$variable = array('VAR_DATA' => $multiline););
$docx->replaceVariableByText($variable, array('parseLineBreaks' =>true));
Regards.