search string for specific word and replace it

[.Ram]

I think it is a regex I need.I have a text-input where users may search my website.They may use the word "\[code\]ELLER\[/code\]" between search phrases, which is in english equal to "\[code\]OR\[/code\]".My search engine however, requires it in english, so I need to replace all ELLER in the query string with OR instead.How can I do this?Btw, it is php...Thanks