The best replacement for amfphp is zend amf.

The best alternatives to amfphp are rated as following:
★★★★★  zend amf
★★★  blazeds
★★★  weborb
  zendamf

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Speaking about amfphp, one probably shouldn't forget about as2.

Other search terms related to amfphp and their ratings are:
★★★★★ as2
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Google Trends

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amfphp
zend amf
blazeds