documentclass[10pt]{article}
setlengthparindent{0pt}
usepackage{hyperref}
usepackage[square,numbers]{natbib}

begin{document}

A single citation: cite{M} \
Multiple citations: cite{M,K,T} \
Page ref. example: citep[p. 10]{B} \
As shown by citet{M,K} or also by citet{B} \

begin{thebibliography}{9}
    bibitem[Me(2006)]{M} Me: My book related to the problem (2006), 145 p.
    % The new line is important!

    bibitem[Karl(2005)]{K} Karl: Karl's paper published in some minor proceedings,
    a local conferrence organized by his university (2005), 23--25.
    % The new line is important!

    bibitem[Test(2013)]{T} Test. A test paper,
    Conf. of testing (2013), 99.
    % The new line is important!

    bibitem[Someone(2013)]{B} Someone, a nice book,
    A good publisher (2013).
    % The new line is important!

end{thebibliography}
end{document}

A single citation: [1]
Multiple citations: [1, 2, 3]
Page ref. example: [4, p. 10]
As shown by Me [1], Karl [2] or also by Someone [4]

letoldcitecite
renewcommand{cite}[1]{tooltip{oldcite{#1}}{#1}}

letoldCommandInQuestionCommandInQuestion
renewcommand{CommandInQuestion}{tooltip{oldCommandInQuestion{#1}}{#1}}