title: Dynamic headers in rmarkdown pdf output
body: <p>I'm creating reports using what I think is a combination of Rmarkdown/Knitr, YAML, LaTeX and Pandoc (I don't fully understand which parts of my code pertain to what because I'm not well versed in anything except R).</p>
  
  <p>I want to know how to add a "dynamic" header, where the contents of the header can be called from variables I read in, for eg., a value from a dataframe I import into the R session. </p>
  
  <p>Specifically, I want to add, say, a unique ID# for every report, to be printed on every page of the report (not necessarily in the header), with a <strong>new</strong> unique ID# for every new report I generate, without having to manually change this information in the header section each time I make a new report.</p>
  
  <p>This is my current header:</p>
  
  <pre><code>header-includes: 
     \usepackage{fancyhdr} 
     \usepackage{graphicx}
     \usepackage{eurosym} 
     \usepackage{booktabs} 
     \pagestyle{fancy} 
     \fancyhf{} 
     \addtolength{\headheight}{1.0cm}
     \rfoot{Page \thepage} 
     \usepackage{setspace}
     \setlength{\topsep}{0pt}
     \fancypagestyle{plain}{\pagestyle{fancy}}
     \renewcommand{\footrulewidth}{0.4pt}
     \iffalse
     \fi
     \rhead{\fontsize{28}{12}\selectfont My Report} 
     \lhead{\includegraphics[width=6cm]{path/to/my/png.png}} 
      \cfoot{some information about footer goes here }
      \usepackage{floatrow}
      \floatsetup[table]{capposition=top, objectset=centering}
      \setlength\parindent{0 pt}\setlength{\parskip}{0.0pt plus 0.0pt}
  output: 
    pdf_document: 
      keep_tex: yes
      latex_engine: xelatex
  geometry: "left=2cm,right=2cm,top=2.5cm,bottom=3cm"
  mainfont: Calibri
  fontsize: 12pt
  </code></pre>