diff --git a/ext/standard/tests/url/gh12703.phpt b/ext/standard/tests/url/gh12703.phpt
new file mode 100644
index 0000000000000..95a5e151c09d2
--- /dev/null
+++ b/ext/standard/tests/url/gh12703.phpt
@@ -0,0 +1,104 @@
+--TEST--
+GH-12703 (parse_url mishandles colon inside path)
+--FILE--
+<?php
+// Case 1 (issue report): an absolute path with a colon and no query string
+// used to return false instead of the path.
+var_dump(parse_url('/page:1'));
+var_dump(parse_url('/page:1', PHP_URL_PATH));
+var_dump(parse_url('/page:1', PHP_URL_SCHEME));
+
+// A query string already worked via a different branch, keep it as a regression guard.
+var_dump(parse_url('/page:1?foo=bar'));
+
+// Pathological single-slash inputs that exercise the new branch path.
+var_dump(parse_url('/:'));
+var_dump(parse_url('/:80'));
+
+// Case 2: a relative-scheme URL (//host/path) with colon-like digits inside
+// the path used to strip the digits out as a phantom port. The host has no
+// explicit port in these inputs, so parse_url should not report one.
+var_dump(parse_url('//www.example.com/foo:65535/'));
+var_dump(parse_url('//www.example.com/foo:1/'));
+var_dump(parse_url('//www.example.com/foo:65536/'));
+var_dump(parse_url('//host/a:1/b:2/'));
+
+// Explicit host port must still be extracted, and the colon inside the path
+// must stay inside the path.
+var_dump(parse_url('//www.example.com:8080/foo:65535/'));
+
+// Full URL with scheme, auth, host, port, path with colon, query, fragment
+// must still round-trip correctly.
+var_dump(parse_url('scheme://user:pass@host:8080/a:1/b:2?q=1#f'));
+?>
+--EXPECT--
+array(1) {
+  ["path"]=>
+  string(7) "/page:1"
+}
+string(7) "/page:1"
+NULL
+array(2) {
+  ["path"]=>
+  string(7) "/page:1"
+  ["query"]=>
+  string(7) "foo=bar"
+}
+array(1) {
+  ["path"]=>
+  string(2) "/:"
+}
+array(1) {
+  ["path"]=>
+  string(4) "/:80"
+}
+array(2) {
+  ["host"]=>
+  string(15) "www.example.com"
+  ["path"]=>
+  string(11) "/foo:65535/"
+}
+array(2) {
+  ["host"]=>
+  string(15) "www.example.com"
+  ["path"]=>
+  string(7) "/foo:1/"
+}
+array(2) {
+  ["host"]=>
+  string(15) "www.example.com"
+  ["path"]=>
+  string(11) "/foo:65536/"
+}
+array(2) {
+  ["host"]=>
+  string(4) "host"
+  ["path"]=>
+  string(9) "/a:1/b:2/"
+}
+array(3) {
+  ["host"]=>
+  string(15) "www.example.com"
+  ["port"]=>
+  int(8080)
+  ["path"]=>
+  string(11) "/foo:65535/"
+}
+array(8) {
+  ["scheme"]=>
+  string(6) "scheme"
+  ["host"]=>
+  string(4) "host"
+  ["port"]=>
+  int(8080)
+  ["user"]=>
+  string(4) "user"
+  ["pass"]=>
+  string(4) "pass"
+  ["path"]=>
+  string(8) "/a:1/b:2"
+  ["query"]=>
+  string(3) "q=1"
+  ["fragment"]=>
+  string(1) "f"
+}
diff --git a/ext/standard/url.c b/ext/standard/url.c
index 4ddf7f80c64f9..cfc7a74d1c4be 100644
--- a/ext/standard/url.c
+++ b/ext/standard/url.c
@@ -54,9 +54,15 @@ static void php_replace_controlchars(char *str, size_t len)
 
 	ZEND_ASSERT(str != NULL);
 
+	/* Replace ASCII C0 control chars (0x00..0x1F) and DEL (0x7F). An inline
+	 * comparison is used instead of iscntrl() because (a) it avoids the
+	 * per-byte locale lookup through __ctype_b_loc(), and (b) URL components
+	 * are bytes, not locale-dependent text, so the C-locale semantics are
+	 * what we want regardless of the process locale. The compiler can also
+	 * auto-vectorize this simple form. */
 	while (s < e) {
-		if (iscntrl(*s)) {
-			*s='_';
+		if (UNEXPECTED(*s < 0x20 || *s == 0x7f)) {
+			*s = '_';
 		}
 		s++;
 	}
@@ -104,7 +110,7 @@ PHPAPI php_url *php_url_parse_ex2(char const *str, size_t length, bool *has_port
 		while (p < e) {
 			/* scheme = 1*[ lowalpha | digit | "+" | "-" | "." ] */
 			if (!isalpha(*p) && !isdigit(*p) && *p != '+' && *p != '.' && *p != '-') {
-				if (e + 1 < ue && e < binary_strcspn(s, ue, "?#")) {
+				if (*s != '/' && e + 1 < ue && e < binary_strcspn(s, ue, "?#")) {
 					goto parse_port;
 				} else if (s + 1 < ue && *s == '/' && *(s + 1) == '/') { /* relative-scheme URL */
 					s += 2;