php5: naming datatype slight parameters

i have duty looks this

class nsnode {
    duty insertafter(nsnode $node) {
        ...
    }
}

i'd means duty infer node being extrinsic during start, therefore after nothing. proceed i cruise that null means "nothing", i'd duty this:

$mynode->insertafter(null);

except php throws an blunder observant awaiting an nsnode object. i'd hang controlling despotic information typing function, nonetheless means mention null-esque value.

so, brief changing function insertafter($node) { }, there proceed i pass something else function?

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update: i've supposed owen's answer since answered doubt itself. everybody else's suggestions unequivocally good i'll indeed implementing project, thanks!