Another picture of how the xlsx template looks before using "replaceVariableText" would have been helpful.
This set the identifier for the Variables I want to replace:
$xlsx->setTemplateSymbol('#');
$xlsx->replaceVariableText(array('VAR_1' => 'New value 1', 'VAR_2' => 'New value 2''),
array('target' => 'sheets'));
Excel Template before the code (set in any Cell):
#VAR_1#
#VAR_2#
Excel Template after the code:
New value 1
New value 2