magic_quotes_gpc is a php.ini setting to automatically add backslash escapes to certain characters in input values (from GPC = Get, Post, Cookie). It’s been long deprecated in PHP, back as far as 5.x I think.
I guess either your theme or a slider plugin is calling this because WordPress core doesn’t (except one reference in the included ID3). I can’t imagine why the functionality would break your layout, so I guess it’s the printing the warning that is.
The best solution is to work out whether the warning is coming from your theme or a plugin – it should be obvious from the file and line number printed – and contact the author of that for an update. However if you want to fix this yourself, I’d
- double-check that you don’t have
magic_quotes_gpc = onin your php.ini file, i.e. that you aren’t actually using it (although I don’t think it works anymore anyway) - replace the calls to
get_magic_quotes_gpc()in the relevant code with just 0, which has been the default in many PHP versions now.