Searching a string for a lot of characters...

[The1corrupted]

Alright, on certain site, I've seen the messages that say "No spaces in the username/password" Or "Illegal characters: '/,\,&,%,$,#,@ (etc.)'" and I was wondering how that's done because as you can see, it has to search for many characters. (data is posted, then these messages appear)

[ManWithNoName]

Sounds like a job for regEx, have a look at the "preg" functions, start maybe with
preg_match + the regex OR operator ( "|" )

see http://php.net/preg_match

[coffear]

As suggested by ManWithNoName they will be using preg_match or some other regular expression based function.

However they won't be checking for all characters that cannot be used they will be checking if the string only contains characters that can be used. A username for example may only allow letters and numbers so they could use the following regular expression:-

'/[a-zA-Z0-9]+/'

If it fails then they are using an illegal character. Whenever you do such checking you should check for allowed characters rather then disallowed characters otherwise you have to go through everything that is not allowed and there is much more scope of missing something.