2.2 Decoding UTF-16
   Decoding of a single character from UTF-16 to an ISO 10646 character
   value proceeds as follows. Let W1 be the next 16-bit integer in the
   sequence of integers representing the text. Let W2 be the (eventual)
   next integer following W1.
   1) If W1 < 0xD800 or W1 > 0xDFFF, the character value U is the value
      of W1. Terminate.
   **2) Determine if W1 is between 0xD800 and 0xDBFF. If not, the sequence
      is in error and no valid character can be obtained using W1.
      Terminate.**
   3) If there is no W2 (that is, the sequence ends with W1), or if W2
      is not between 0xDC00 and 0xDFFF, the sequence is in error.
      Terminate.
   4) Construct a 20-bit unsigned integer U', taking the 10 low-order
      bits of W1 as its 10 high-order bits and the 10 low-order bits of
      W2 as its 10 low-order bits.

/home/nikic/php-7.4/ext/mbstring/mbstring.c: In function ‘mbfl_split_output’:
/home/nikic/php-7.4/ext/mbstring/mbstring.c:2308:54: warning: pointer targets in passing argument 2 of ‘add_next_index_stringl’ differ in signedness [-Wpointer-sign]
         add_next_index_stringl(params->return_value, chunk->val, chunk->len); /* add chunk to the array */
                                                      ^~~~~
In file included from /home/nikic/php-7.4/main/php.h:37:0,
                 from /home/nikic/php-7.4/ext/mbstring/mbstring.c:27:
/home/nikic/php-7.4/Zend/zend_API.h:429:14: note: expected ‘const char *’ but argument is of type ‘unsigned char *’
 ZEND_API int add_next_index_stringl(zval *arg, const char *str, size_t length);
              ^~~~~~~~~~~~~~~~~~~~~~
/home/nikic/php-7.4/ext/mbstring/mbstring.c: In function ‘zif_mb_str_split’:
/home/nikic/php-7.4/ext/mbstring/mbstring.c:2384:35: warning: pointer targets in initialization differ in signedness [-Wpointer-sign]
             char const *chunk_p = p; /* chunk first byte pointer */
                                   ^
/home/nikic/php-7.4/ext/mbstring/mbstring.c:2465:50: warning: pointer targets in passing argument 2 of ‘add_next_index_stringl’ differ in signedness [-Wpointer-sign]
             add_next_index_stringl(return_value, p, chunk_len);
                                                  ^
In file included from /home/nikic/php-7.4/main/php.h:37:0,
                 from /home/nikic/php-7.4/ext/mbstring/mbstring.c:27:
/home/nikic/php-7.4/Zend/zend_API.h:429:14: note: expected ‘const char *’ but argument is of type ‘const unsigned char *’
 ZEND_API int add_next_index_stringl(zval *arg, const char *str, size_t length);
              ^~~~~~~~~~~~~~~~~~~~~~
/home/nikic/php-7.4/ext/mbstring/mbstring.c:2467:46: warning: pointer targets in passing argument 2 of ‘add_next_index_stringl’ differ in signedness [-Wpointer-sign]
         add_next_index_stringl(return_value, p, last - p);
                                              ^
In file included from /home/nikic/php-7.4/main/php.h:37:0,
                 from /home/nikic/php-7.4/ext/mbstring/mbstring.c:27:
/home/nikic/php-7.4/Zend/zend_API.h:429:14: note: expected ‘const char *’ but argument is of type ‘const unsigned char *’
 ZEND_API int add_next_index_stringl(zval *arg, const char *str, size_t length);
              ^~~~~~~~~~~~~~~~~~~~~~