diff --git a/ext/mbstring/mbstring.c b/ext/mbstring/mbstring.c
index 1d5c27a2a3815..5839945d47572 100644
--- a/ext/mbstring/mbstring.c
+++ b/ext/mbstring/mbstring.c
@@ -6395,13 +6395,15 @@ static zend_string* mb_mime_header_decode(zend_string *input, const mbfl_encodin
 					p = temp;
 					/* Decoding of MIME encoded word was successful;
 					 * Try to collapse a run of whitespace */
-					if (p < e && (*p == '\n' || *p == '\r')) {
+					if (p < e && (*p == '\n' || *p == '\r' || *p == '\t' || *p == ' ')) {
 						do {
 							p++;
 						} while (p < e && (*p == '\n' || *p == '\r' || *p == '\t' || *p == ' '));
 						/* We will only actually output a space if this is not immediately followed
 						 * by another valid encoded word */
 						space_pending = true;
+					} else {
+						space_pending = false;
 					}
 					continue;
 				}
diff --git a/ext/mbstring/tests/gh20674.phpt b/ext/mbstring/tests/gh20674.phpt
new file mode 100644
index 0000000000000..2fb8206037dee
--- /dev/null
+++ b/ext/mbstring/tests/gh20674.phpt
@@ -0,0 +1,40 @@
+--TEST--
+GH-20674 (mb_decode_mimeheader does not handle separator)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+
+$subject = '=?us-ascii?Q?The_PH?=  =?us-ascii?Q?P_8.5?=';
+var_dump(mb_decode_mimeheader($subject));
+
+// mb_decode_mimeheader's backward compatible for TAB(\t)
+$subject = "=?us-ascii?Q?The_PH?=\t=?us-ascii?Q?P_8.5?=";
+var_dump(mb_decode_mimeheader($subject));
+
+$subject = "=?us-ascii?Q?The_PH?=\t =?us-ascii?Q?P_8.5?=";
+var_dump(mb_decode_mimeheader($subject));
+
+$subject = "=?us-ascii?Q?The_PH?= \t =?us-ascii?Q?P_8.5?=";
+var_dump(mb_decode_mimeheader($subject));
+
+// from RFC 2047 https://www.ietf.org/rfc/rfc2047#section-8
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?= b)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a_b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?= =?ISO-8859-1?Q?b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=  =?ISO-8859-1?Q?b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=
+	=?ISO-8859-1?Q?b?=)"));
+?>
+--EXPECTF--
+string(11) "The PHP 8.5"
+string(11) "The PHP 8.5"
+string(11) "The PHP 8.5"
+string(11) "The PHP 8.5"
+string(3) "(a)"
+string(5) "(a b)"
+string(5) "(a b)"
+string(4) "(ab)"
+string(4) "(ab)"
+string(4) "(ab)"