[php] fix 'mixed' invalid for settype()

[NickUfer]

Fixes: #10575

Adds an if statement in ObjectSerializer#deserialize for the mixed openapi type which resolves the correct class for the supplied data.

@jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @ybelenko @renepardon

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[wing328]

LGTM but I've not tested it locally.