I have this without any issues if I use it alone. However if I call it with input or include I get this 
Why?
I have this without any issues if I use it alone. However if I call it with input or include I get this 
Why?
Let's say, the referenced answer is in file answer.tex. Then a minimal main document would be:
\documentclass{article}
\usepackage{xcolor}
\begin{document}
\input{answer}
\end{document}
That generates tons of errors. let's ignore them with \batchmode (or pressing q at the first error message). The result is:
What happened, you have tried to input a complete LaTeX document inside the body of another. This is not allowed, already the first error messages say this:
! LaTeX Error: Can be used only in preamble.
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.1 \documentclass
{article}
The PDF output (image above) is the result of LaTeX's/TeX's error recovery – put a frame around, then you can call it "Modern Art". :-)
A preamble (the stuff before \begin{document}) cannot be called after \begin{document}. But you can merge the two preambles. For example, if you need package listings for the second document, but the first document has not loaded it, then add
\usepackage{listings}
to the preamble of the main document.
The same applies to the color definitions (\definecolor lines) and the settings (\lstset) for package listings. Put them into the preamble of the main document.
Then in the body (between \begin{document} and \end{document}
you can put the body or parts of it of the second document via copy-paste, \input or \include. The latter can only be used for parts of the document that start and ends at page boundaries (e.g. chapters) and cannot be nested.
A scheme for the new main document would be:
\documentclass and packages.\usepackage{listings})\lstset)\begin{document}\begin{lstlistings}...\end{lstlistings} with the PHP code inbetween.\lstinputlisting (see the documentation of listings)\begin{document} and \end{document} and excluding these markers) can be put into a file that is
included via \input.\include does not make sense here.)\end{document}I hope, it helps a little or specify, what you want to achieve.