Based on the following upstream commits:

https://github.com/php/php-src/commit/8d35a423838eb462cd39ee535c5d003073cc5f22
https://github.com/php/php-src/commit/d8200e48857aeaf09f7127751efc5632ef7660a7
https://github.com/php/php-src/commit/9a8e7b571896f39f83cb000fdb7c466b6d328d52

--- a/ext/intl/breakiterator/breakiterator_class.cpp
+++ b/ext/intl/breakiterator/breakiterator_class.cpp
@@ -38,6 +38,7 @@ extern "C" {
 }
 
 using PHP::CodePointBreakIterator;
+using icu::RuleBasedBreakIterator;
 
 /* {{{ Global variables */
 zend_class_entry *BreakIterator_ce_ptr;
--- a/ext/intl/breakiterator/breakiterator_class.h
+++ b/ext/intl/breakiterator/breakiterator_class.h
@@ -26,6 +26,8 @@
 
 #ifndef USE_BREAKITERATOR_POINTER
 typedef void BreakIterator;
+#else
+using icu::BreakIterator;
 #endif
 
 typedef struct {
--- a/ext/intl/breakiterator/breakiterator_methods.cpp
+++ b/ext/intl/breakiterator/breakiterator_methods.cpp
@@ -32,6 +32,8 @@ extern "C" {
 }
 
 using PHP::CodePointBreakIterator;
+using icu::BreakIterator;
+using icu::Locale;
 
 U_CFUNC PHP_METHOD(BreakIterator, __construct)
 {
--- a/ext/intl/breakiterator/codepointiterator_internal.cpp
+++ b/ext/intl/breakiterator/codepointiterator_internal.cpp
@@ -33,6 +33,8 @@ typedef union {
 
 using namespace PHP;
 
+using icu::UCharCharacterIterator;
+
 UOBJECT_DEFINE_RTTI_IMPLEMENTATION(CodePointBreakIterator);
 
 CodePointBreakIterator::CodePointBreakIterator()
--- a/ext/intl/breakiterator/codepointiterator_internal.h
+++ b/ext/intl/breakiterator/codepointiterator_internal.h
@@ -18,8 +18,11 @@
 #define CODEPOINTITERATOR_INTERNAL_H
 
 #include <unicode/brkiter.h>
+#include <unicode/unistr.h>
 
-using U_ICU_NAMESPACE::BreakIterator;
+using icu::BreakIterator;
+using icu::CharacterIterator;
+using icu::UnicodeString;
 
 namespace PHP {
 
--- a/ext/intl/breakiterator/rulebasedbreakiterator_methods.cpp
+++ b/ext/intl/breakiterator/rulebasedbreakiterator_methods.cpp
@@ -26,6 +26,9 @@ extern "C" {
 #include "../intl_convertcpp.h"
 #include "../intl_common.h"
 
+using icu::RuleBasedBreakIterator;
+using icu::Locale;
+
 static inline RuleBasedBreakIterator *fetch_rbbi(BreakIterator_object *bio) {
 	return (RuleBasedBreakIterator*)bio->biter;
 }
--- a/ext/intl/calendar/calendar_class.cpp
+++ b/ext/intl/calendar/calendar_class.cpp
@@ -34,6 +34,9 @@ extern "C" {
 #include <assert.h>
 }
 
+using icu::GregorianCalendar;
+using icu::Locale;
+
 /* {{{ Global variables */
 zend_class_entry *Calendar_ce_ptr;
 zend_class_entry *GregorianCalendar_ce_ptr;
--- a/ext/intl/calendar/calendar_class.h
+++ b/ext/intl/calendar/calendar_class.h
@@ -26,6 +26,8 @@
 
 #ifndef USE_CALENDAR_POINTER
 typedef void Calendar;
+#else
+using icu::Calendar;
 #endif
 
 typedef struct {
--- a/ext/intl/calendar/calendar_methods.cpp
+++ b/ext/intl/calendar/calendar_methods.cpp
@@ -40,6 +40,8 @@ extern "C" {
 }
 #include "../common/common_enum.h"
 
+using icu::Locale;
+
 U_CFUNC PHP_METHOD(IntlCalendar, __construct)
 {
 	zend_throw_exception( NULL,
--- a/ext/intl/calendar/gregoriancalendar_methods.cpp
+++ b/ext/intl/calendar/gregoriancalendar_methods.cpp
@@ -23,6 +23,8 @@
 #include <unicode/locid.h>
 #include <unicode/calendar.h>
 #include <unicode/gregocal.h>
+#include <unicode/ustring.h>
+
 extern "C" {
 #include "../php_intl.h"
 #include "../intl_common.h"
@@ -34,6 +36,11 @@ extern "C" {
 #include "zend_exceptions.h"
 }
 
+using icu::GregorianCalendar;
+using icu::Locale;
+using icu::UnicodeString;
+using icu::StringPiece;
+
 static inline GregorianCalendar *fetch_greg(Calendar_object *co) {
 	return (GregorianCalendar*)co->ucal;
 }
--- a/ext/intl/common/common_date.cpp
+++ b/ext/intl/common/common_date.cpp
@@ -27,6 +27,9 @@ extern "C" {
 
 #include "zend_portability.h"
 
+using icu::TimeZone;
+using icu::UnicodeString;
+
 /* {{{ timezone_convert_datetimezone
  *      The timezone in DateTime and DateTimeZone is not unified. */
 U_CFUNC TimeZone *timezone_convert_datetimezone(int type,
--- a/ext/intl/common/common_date.h
+++ b/ext/intl/common/common_date.h
@@ -28,6 +28,8 @@ U_CDECL_END
 
 #include <unicode/timezone.h>
 
+using icu::TimeZone;
+
 U_CFUNC TimeZone *timezone_convert_datetimezone(int type, void *object, int is_datetime, intl_error *outside_error, const char *func);
 U_CFUNC int intl_datetime_decompose(zval *z, double *millis, TimeZone **tz,
 		intl_error *err, const char *func);
--- a/ext/intl/common/common_enum.h
+++ b/ext/intl/common/common_enum.h
@@ -75,6 +75,7 @@ U_CFUNC zval *zoi_with_current_get_current_data(zend_object_iterator *iter);
 U_CFUNC void zoi_with_current_invalidate_current(zend_object_iterator *iter);
 
 #ifdef __cplusplus
+using icu::StringEnumeration;
 U_CFUNC void IntlIterator_from_StringEnumeration(StringEnumeration *se, zval *object);
 #endif
 
--- a/ext/intl/converter/converter.c
+++ b/ext/intl/converter/converter.c
@@ -18,6 +18,8 @@
 #include "zend_exceptions.h"
 
 #include <unicode/utypes.h>
+#include <unicode/utf8.h>
+#include <unicode/utf16.h>
 #include <unicode/ucnv.h>
 #include <unicode/ustring.h>
 
--- a/ext/intl/dateformat/dateformat_format_object.cpp
+++ b/ext/intl/dateformat/dateformat_format_object.cpp
@@ -33,6 +33,12 @@ extern "C" {
 #include "../common/common_date.h"
 }
 
+using icu::Locale;
+using icu::DateFormat;
+using icu::GregorianCalendar;
+using icu::StringPiece;
+using icu::SimpleDateFormat;
+
 static const DateFormat::EStyle valid_styles[] = {
 		DateFormat::kNone,
 		DateFormat::kFull,
--- a/ext/intl/dateformat/dateformat_helpers.cpp
+++ b/ext/intl/dateformat/dateformat_helpers.cpp
@@ -28,6 +28,8 @@ extern "C" {
 #include "../calendar/calendar_class.h"
 }
 
+using icu::GregorianCalendar;
+
 int datefmt_process_calendar_arg(zval* calendar_zv,
 								 Locale const& locale,
 								 const char *func_name,
--- a/ext/intl/dateformat/dateformat_helpers.h
+++ b/ext/intl/dateformat/dateformat_helpers.h
@@ -22,11 +22,16 @@
 #endif
 
 #include <unicode/calendar.h>
+#include <unicode/datefmt.h>
 
 extern "C" {
 #include "../php_intl.h"
 }
 
+using icu::Locale;
+using icu::Calendar;
+using icu::DateFormat;
+
 int datefmt_process_calendar_arg(zval* calendar_zv,
 								 Locale const& locale,
 								 const char *func_name,
--- a/ext/intl/grapheme/grapheme_string.c
+++ b/ext/intl/grapheme/grapheme_string.c
@@ -24,6 +24,7 @@
 #include "grapheme_util.h"
 
 #include <unicode/utypes.h>
+#include <unicode/utf8.h>
 #include <unicode/ucol.h>
 #include <unicode/ustring.h>
 #include <unicode/ubrk.h>
@@ -834,10 +835,10 @@ PHP_FUNCTION(grapheme_extract)
 	pstr = str + start;
 
 	/* just in case pstr points in the middle of a character, move forward to the start of the next char */
-	if ( !UTF8_IS_SINGLE(*pstr) && !U8_IS_LEAD(*pstr) ) {
-		char *str_end = str + str_len;
+	if ( !U8_IS_SINGLE(*pstr) && !U8_IS_LEAD(*pstr) ) {
+		unsigned char *str_end = str + str_len;
 
-		while ( !UTF8_IS_SINGLE(*pstr) && !U8_IS_LEAD(*pstr) ) {
+		while ( !U8_IS_SINGLE(*pstr) && !U8_IS_LEAD(*pstr) ) {
 			pstr++;
 			if ( pstr >= str_end ) {
 				intl_error_set( NULL, U_ILLEGAL_ARGUMENT_ERROR,
--- a/ext/intl/intl_convertcpp.h
+++ b/ext/intl/intl_convertcpp.h
@@ -26,6 +26,8 @@
 #include <unicode/unistr.h>
 #include <zend_types.h>
 
+using icu::UnicodeString;
+
 int intl_stringFromChar(UnicodeString &ret, char *str, size_t str_len, UErrorCode *status);
 
 zend_string* intl_charFromString(const UnicodeString &from, UErrorCode *status);
--- a/ext/intl/msgformat/msgformat_helpers.cpp
+++ b/ext/intl/msgformat/msgformat_helpers.cpp
@@ -78,6 +78,16 @@ MessageFormatAdapter::getMessagePattern(MessageFormat* m) {
 #endif
 U_NAMESPACE_END
 
+using icu::Formattable;
+using icu::Format;
+using icu::DateFormat;
+using icu::MessageFormat;
+#ifdef HAS_MESSAGE_PATTERN
+using icu::MessagePattern;
+#endif
+using icu::MessageFormatAdapter;
+using icu::FieldPosition;
+
 U_CFUNC int32_t umsg_format_arg_count(UMessageFormat *fmt)
 {
 	int32_t fmt_count = 0;
@@ -229,15 +239,16 @@ static HashTable *umsg_parse_format(MessageFormatter_object *mfo,
 					UnicodeString typeString = mp.getSubstring(type_part);
 					/* This is all based on the rules in the docs for MessageFormat
 					 * @see http://icu-project.org/apiref/icu4c/classMessageFormat.html */
-					if (typeString == "number") {
+#define ASCII_LITERAL(s) UNICODE_STRING(s, sizeof(s)-1)
+					if (typeString == ASCII_LITERAL("number")) {
 						MessagePattern::Part style_part = mp.getPart(i + 1); /* Not advancing i */
 						if (style_part.getType() == UMSGPAT_PART_TYPE_ARG_STYLE) {
 							UnicodeString styleString = mp.getSubstring(style_part);
-							if (styleString == "integer") {
+							if (styleString == ASCII_LITERAL("integer")) {
 								type = Formattable::kInt64;
-							} else if (styleString == "currency") {
+							} else if (styleString == ASCII_LITERAL("currency")) {
 								type = Formattable::kDouble;
-							} else if (styleString == "percent") {
+							} else if (styleString == ASCII_LITERAL("percent")) {
 								type = Formattable::kDouble;
 							} else { /* some style invalid/unknown to us */
 								type = Formattable::kDouble;
@@ -245,12 +256,13 @@ static HashTable *umsg_parse_format(MessageFormatter_object *mfo,
 						} else { // if missing style, part, make it a double
 							type = Formattable::kDouble;
 						}
-					} else if ((typeString == "date") || (typeString == "time")) {
+					} else if ((typeString == ASCII_LITERAL("date")) || (typeString == ASCII_LITERAL("time"))) {
 						type = Formattable::kDate;
-					} else if ((typeString == "spellout") || (typeString == "ordinal")
-							|| (typeString == "duration")) {
+					} else if ((typeString == ASCII_LITERAL("spellout")) || (typeString == ASCII_LITERAL("ordinal"))
+							|| (typeString == ASCII_LITERAL("duration"))) {
 						type = Formattable::kDouble;
 					}
+#undef ASCII_LITERAL
 				} else {
 					/* If there's no UMSGPAT_PART_TYPE_ARG_TYPE right after a
 					 * UMSGPAT_ARG_TYPE_SIMPLE argument, then the pattern
--- a/ext/intl/normalizer/normalizer_normalize.c
+++ b/ext/intl/normalizer/normalizer_normalize.c
@@ -24,6 +24,7 @@
 #include "normalizer_class.h"
 #include "normalizer_normalize.h"
 #include "intl_convert.h"
+#include <unicode/utf8.h>
 
 /* {{{ proto string Normalizer::normalize( string $input [, string $form = FORM_C] )
  * Normalize a string. }}} */
--- a/ext/intl/timezone/timezone_class.cpp
+++ b/ext/intl/timezone/timezone_class.cpp
@@ -37,6 +37,8 @@ extern "C" {
 #include <ext/date/php_date.h>
 }
 
+using icu::Calendar;
+
 /* {{{ Global variables */
 U_CDECL_BEGIN
 zend_class_entry *TimeZone_ce_ptr = NULL;
--- a/ext/intl/timezone/timezone_class.h
+++ b/ext/intl/timezone/timezone_class.h
@@ -29,6 +29,8 @@
 
 #ifndef USE_TIMEZONE_POINTER
 typedef void TimeZone;
+#else
+using icu::TimeZone;
 #endif
 
 typedef struct {
--- a/ext/intl/timezone/timezone_methods.cpp
+++ b/ext/intl/timezone/timezone_methods.cpp
@@ -23,6 +23,7 @@
 #include <unicode/locid.h>
 #include <unicode/timezone.h>
 #include <unicode/ustring.h>
+#include <unicode/calendar.h>
 #include "intl_convertcpp.h"
 
 #include "../common/common_date.h"
@@ -37,6 +38,9 @@ extern "C" {
 }
 #include "common/common_enum.h"
 
+using icu::Locale;
+using icu::Calendar;
+
 U_CFUNC PHP_METHOD(IntlTimeZone, __construct)
 {
 	zend_throw_exception( NULL,
--- a/ext/intl/uchar/uchar.c
+++ b/ext/intl/uchar/uchar.c
@@ -3,6 +3,7 @@
 #include "intl_convert.h"
 
 #include <unicode/uchar.h>
+#include <unicode/utf8.h>
 
 #define IC_METHOD(mname) PHP_METHOD(IntlChar, mname)
 
-- 
2.19.2