regular expression for (NOT 'word')

how would i build a regular expression in php that would match everything but the given word. For example, match the string only if there isnt a word 'php' in the string. I understand i can list characters i dont want to see in the string: [^php]. but this means there cant be any p or h in the string. And ^(php) checks if the string starts with 'php'. How should i do it?