Test string:
$s = "convert this: ";
$s .= "–, —, †, ‡, •, ≤, ≥, μ, ₪, ©, ® y ™, ⅓, ⅔, ⅛, ⅜, ⅝, ⅞, ™, Ω, ℮, ∑, ⌂, ♀, ♂ ";
$s .= "but, not convert ordinary characters to entities";
Test string:
$s = "convert this: ";
$s .= "–, —, †, ‡, •, ≤, ≥, μ, ₪, ©, ® y ™, ⅓, ⅔, ⅛, ⅜, ⅝, ⅞, ™, Ω, ℮, ∑, ⌂, ♀, ♂ ";
$s .= "but, not convert ordinary characters to entities";
asssuming your input string is UTF-8, this should encode most everything into numeric entities.
Well htmlentities doesn't work correctly. Fortunately someone has posted code on the php website that seems to do the translation of multibyte characters properly
I did work on decoding ascii into html coded text (&#xxxx). https://github.com/hellonearthis/ascii2web