in many ways, WP exemplifies PHP core devs' philosophy of "dont ever break existing ecosystem compat". PHP has some major WTF that lingers on because of this. i'm very surprised that it has been progressing at all considering how stubborn they are on even fixing completely obvious shit like "unexpected comma" errors written in Hebrew. wordpress takes this attitude to a whole new level :(
there is great, maintainable software written in clean* PHP; the only thing i've learned from WP is how not to write software.