Description:
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It would be quite helpful to have a helper function in_string(), like in_array(), 
that checks if a substring is within some string. Sure, with strpos()/stripos(), 
this can be achieved, but not everyone understands that immediately (and the same 
could be said about in_array(), which can also be done quite simply with a 
foreach, but surely not everyone gets that). There's some code how that might 
work in PHP at MediaWiki 
http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/includes/GlobalFunctions.p
hp?view=markup which is open-source and could be used.

Now it is the question if the third parameter should also be in PHP, but no 
matter which way, it would still probably not too difficult to create such a 
helper function.

Test script:
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# MediaWiki's way of doing this

function in_string( $needle, $str, $insensitive = false ) {
    $func = 'strpos';
    if( $insensitive ) $func = 'stripos';
    return $func( $str, $needle ) !== false;
}

# don't use (bool)$func($str, $needle) because it can be 0 (first character)