Explode a string on upper case characters?

Ho can i explode $param string into $chunks pieces based on upper-case characters?

$string = 'setIfUnmodifiedSince';
$method = substr($string, 0, 3);
$param  = substr($string, 3);

// Split $param and implode with '-' separator
$chunks = splitAtUpperCase($param); // Chunks are: 'If', 'Unmodified' and 'Since'
$field  = implode('-', $chunks); // Get If-Unmodified-Since HTTP field name

Answer

Using preg_split() on [A-Z] should do:

function splitAtUpperCase($s) {
        return preg_split('/(?=[A-Z])/', $s, -1, PREG_SPLIT_NO_EMPTY);
}

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EDIT
If you don’t need the array itself, you can just preprend uppercase characters (except the first) with a hyphen (-):

preg_replace('/(?<!^)([A-Z])/', '-\1', $param);

(demo)