diff --git a/ext/mbstring/libmbfl/mbfl/mbfl_encoding.c b/ext/mbstring/libmbfl/mbfl/mbfl_encoding.c
index f79d21922508e..6e44f68b85790 100644
--- a/ext/mbstring/libmbfl/mbfl/mbfl_encoding.c
+++ b/ext/mbstring/libmbfl/mbfl/mbfl_encoding.c
@@ -349,7 +349,7 @@ const mbfl_encoding *mbfl_name2encoding_ex(const char *name, size_t name_len)
 	/* search MIME charset name */
 	for (encoding = mbfl_encoding_ptr_list; *encoding; encoding++) {
 		if ((*encoding)->mime_name) {
-			if (strcasecmp((*encoding)->mime_name, name) == 0) {
+			if (strncasecmp((*encoding)->mime_name, name, name_len) == 0 && (*encoding)->mime_name[name_len] == '\0') {
 				return *encoding;
 			}
 		}
@@ -359,7 +359,7 @@ const mbfl_encoding *mbfl_name2encoding_ex(const char *name, size_t name_len)
 	for (encoding = mbfl_encoding_ptr_list; *encoding; encoding++) {
 		if ((*encoding)->aliases) {
 			for (const char **alias = (*encoding)->aliases; *alias; alias++) {
-				if (strcasecmp(*alias, name) == 0) {
+				if (strncasecmp(name, *alias, name_len) == 0 && (*alias)[name_len] == '\0') {
 					return *encoding;
 				}
 			}
diff --git a/ext/mbstring/tests/gh15824.phpt b/ext/mbstring/tests/gh15824.phpt
new file mode 100644
index 0000000000000..570bb2b9d0382
--- /dev/null
+++ b/ext/mbstring/tests/gh15824.phpt
@@ -0,0 +1,37 @@
+--TEST--
+GH-15824 (ValueError: mb_detect_encoding(): Argument #2 ($encodings) contains invalid encoding "UTF8")
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+echo "== alias name ==\n";
+var_dump(mb_detect_encoding('abc', 'UTF8, ASCII'));
+var_dump(mb_detect_encoding('こんにちは', 'UTF8, ASCII'));
+var_dump(mb_detect_encoding('こんにちは', 'ASCII, UTF8'));
+var_dump(mb_detect_encoding("\xC5", 'US-ASCII, LATIN4'));
+var_dump(mb_detect_encoding("\xC5", 'US-ASCII, cyrillic'));
+/* 0x9D is not located in CP1254 */
+var_dump(mb_detect_encoding("\x9D", 'US-ASCII, CP1254, cyrillic'));
+var_dump(mb_detect_encoding("\x9D", 'US-ASCII, CP1254, cyrillic', false));
+
+echo "== mime name ==\n";
+var_dump(mb_detect_encoding('abc', 'ANSI_X3.4-1968, ISO-8859-1'));
+var_dump(mb_detect_encoding('abc', 'CP50220, ANSI_X3.4-1968'));
+/* last comma is not mistake, intentionally */
+var_dump(mb_detect_encoding(bin2hex('1b24422422242424262428242a1b2842'), 'CP50220, ANSI_X3.4-1968,', false));
+var_dump(mb_detect_encoding('😄', 'US-ASCII, UTF-8-Mobile#KDDI-B, UTF-8'));
+?>
+--EXPECT--
+== alias name ==
+string(5) "UTF-8"
+string(5) "UTF-8"
+string(5) "UTF-8"
+string(10) "ISO-8859-4"
+string(10) "ISO-8859-5"
+string(10) "ISO-8859-5"
+string(10) "ISO-8859-5"
+== mime name ==
+string(5) "ASCII"
+string(7) "CP50220"
+string(7) "CP50220"
+string(19) "UTF-8-Mobile#KDDI-B"