Is there a difference between the UTC and Etc/UTC time zones?

In the PHP documentation, list of supported time zones, UTC is listed twice:

• UTC
• Etc/UTC

Is there any conceptual difference between those two, or are they just synonyms?

Firstly, to answer the question:
There is NO difference between UTC and Etc/UTC time zones.

Etc/UTC is a timezone in the Olson-timezone-database (tz database), also known as IANA-timezones-database, in which all timezones conform to a uniform naming convention: Area/Location.

Since, some timezones cannot be attributed to any Area of the world (i.e. continents or oceans), the special Area Etc (Etcetera) was introduced. This applies mainly to administrative timezones such as UTC.
Thus, to conform with the naming convention, the universal coordinated time(zone) is named Etc/UTC in the tz database.

For administrative timezones other than UTC (e.g. GMT+4, GMT-8), the tz database uses POSIX-style signs in the zone-names. POSIX has positive signs for zones that are behind Greenwich (west of Greenwich) and negative signs for zones that are ahead of Greenwich (east of Greenwich).

POSIX-style signs in timezones are the opposite of the definition of timezones in the nowadays widespread and mostly used ISO 8601. In the ISO 8601 timezone format, negative signs indicate a zone is behind UTC (west of Greenwich) and positive signs indicate a zone is ahead of UTC (east of Greenwich). This is what has become the standard usage nowadays.

Possible reasons for the opposite definition in POSIX are:

• POSIX is part of UNIX, which was developed in the USA, which is behind UTC (west of Greenwich). The POSIX format allows the US timezones to be represented as EST5, PST8, i.e. omitting the (+) sign.
• Usually, computer programs and operating systems internally do everything in UTC time. With POSIX-style signs you can add time and timezone in order to get UTC time. Example: "03:30 PST8" or "03:30 GMT+8" mean it is "11:30 UTC".

Etc/UTC is the time zone whose display name is UTC. That is, they're long and short names for the same timezone, per IANA's time zone database.

ETC/GMT+4 is the same as GMT-4.

public static void main(String[] args) {
    TimeZone tz = TimeZone.getTimeZone("Etc/GMT-7");
    System.out.println(tz);

    tz = TimeZone.getTimeZone("GMT+7");
    System.out.println(tz);

}

You can test it by yourself.

But I don't know what ETC mean..