ext/intl: Fix grapheme_extract() next offset

When the starting position lies inside a multibyte UTF-8 character,
grapheme_extract() advances the cursor but reports a next offset based on
the original position. Derive the adjusted offset from the cursor for both
extraction paths.

Add regression coverage for extraction modes, ASCII and Unicode tails,
and negative starting offsets, and document the fix in NEWS.

Co-authored-by: David Carlier <devnexen@gmail.com>

Author: LamentXU123

NEWS

@@ -84,6 +84,8 @@ PHP                                                                        NEWS
     haystacks. (Weilin Du)
   . Fixed grapheme_strrpos() and grapheme_strripos() skipping overlapping matches
     when using a negative offset. (Weilin Du)
+  . Fixed grapheme_extract() returning an incorrect next offset when starting
+    inside a multibyte UTF-8 character. (Weilin Du, David Carlier)
 
 - Lexbor:
   . Merge patches lexbor/lexbor@8a14bc0 and lexbor/lexbor@f67ce4b, fixing a

ext/intl/grapheme/grapheme_string.c

@@ -786,7 +786,8 @@ PHP_FUNCTION(grapheme_extract)
 		}
 	}
 
-	str_len -= (pstr - str);
+	lstart = pstr - str;
+	str_len -= lstart;
 
 	/* if the string is all ASCII up to size+1 - or str_len whichever is first - then we are done.
 		(size + 1 because the size-th character might be the beginning of a grapheme cluster)
@@ -795,7 +796,7 @@ PHP_FUNCTION(grapheme_extract)
 	if ( -1 != grapheme_ascii_check((unsigned char *)pstr, MIN(size + 1, str_len)) ) {
 		size_t nsize = MIN(size, str_len);
 		if ( NULL != next ) {
-			ZEND_TRY_ASSIGN_REF_LONG(next, start + nsize);
+			ZEND_TRY_ASSIGN_REF_LONG(next, lstart + nsize);
 		}
 		RETURN_STRINGL(pstr, nsize);
 	}
@@ -829,7 +830,7 @@ PHP_FUNCTION(grapheme_extract)
 	ubrk_close(bi);
 
 	if ( NULL != next ) {
-		ZEND_TRY_ASSIGN_REF_LONG(next, start + ret_pos);
+		ZEND_TRY_ASSIGN_REF_LONG(next, lstart + ret_pos);
 	}
 
 	RETURN_STRINGL(((char *)pstr), ret_pos);

ext/intl/tests/grapheme_extract_next_offset.phpt

@@ -0,0 +1,39 @@
+--TEST--
+grapheme_extract() includes skipped UTF-8 continuation bytes in the next offset
+--EXTENSIONS--
+intl
+--FILE--
+<?php
+
+$cases = [
+    ["\u{00E9}x", 1, 1, GRAPHEME_EXTR_COUNT],
+    ["\u{1F600}x", 1, 1, GRAPHEME_EXTR_COUNT],
+    ["\u{1F600}x", 2, 1, GRAPHEME_EXTR_COUNT],
+    ["\u{1F600}x", 3, 1, GRAPHEME_EXTR_COUNT],
+    ["\u{00E9}\u{1F600}x", 1, 1, GRAPHEME_EXTR_COUNT],
+    ["\u{00E9}\u{1F600}x", 1, 4, GRAPHEME_EXTR_MAXBYTES],
+    ["\u{00E9}\u{1F600}x", 1, 1, GRAPHEME_EXTR_MAXCHARS],
+    ["\u{1F600}\u{00E9}x", -4, 1, GRAPHEME_EXTR_COUNT],
+    ["\u{00E9}x", -2, 1, GRAPHEME_EXTR_COUNT],
+    ["\u{1F600}x", 0, 1, GRAPHEME_EXTR_COUNT],
+    ["\u{00E9}x", 1, 0, GRAPHEME_EXTR_COUNT],
+];
+
+foreach ($cases as [$string, $start, $size, $type]) {
+    $result = grapheme_extract($string, $size, $type, $start, $next);
+    echo bin2hex($result), ' ', $next, "\n";
+}
+
+?>
+--EXPECT--
+78 3
+78 5
+78 5
+78 5
+f09f9880 6
+f09f9880 6
+f09f9880 6
+c3a9 6
+78 3
+f09f9880 4
+ 1