feat(php-shim): implement pure-function todo!() stubs

Fill in the shim functions that have an easy, accurate pure-Rust port:
array (merge/flip/fill_keys/slice/search/is_list), string (substr_count,
strpbrk, raw/urlencode, ucfirst/ucwords, strstr, strcspn, explode limit,
str_replace arrays), mb_* (strlen/substr/str_split), sort callbacks
(uasort/uasort_map/uksort), levenshtein, intdiv, ceil, ctype_digit,
json_encode, count_mixed and the PhpMixed/to_* boolean casts.

Runtime- or crate-dependent and hard-to-port-accurately stubs (sprintf,
version_compare, natural sort, hashing, etc.) are left as todo!().

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