"How can I use fprintf to store a string in a file (( with ITALIC font ))?"
You can't, because textfiles do NOT contain any formatting information.**
You could write a textfile using some standard markup language which includes formatting, e.g. markdown, HTML, etc.
** Just to be absolutely clear: textfiles do NOT store any information about the typeface (font), the fontsize, text color, text bold, text italic, or any other formatting. By definition, textfiles store a long vector of character codes (which are just numbers representing what letter it is... and NOTHING else).
