No, not OK

No, the Z is an offset-from-UTC. So it should not be combined redundantly with a numerical offset of +00:00 or +0000.

ISO 8601

While I do not have access to a paid copy of the ISO 8601 spec, the Wikipedia page clearly states that the Z must follow the time-of-day:

…add a Z directly after the time without a space.

IETF RFC 3339

The freely-available RFC 3339, a profile of ISO 8601, defines a Z as being attached to a time-of-day:

A suffix … applied to a time …

The RFC also states with formal ABNF notation that we should use either a Z or a number. In ABNF, the slash (SOLIDUS) means “or” (exclusive ‘or’), while the pair of square brackets means “optional”.

time-numoffset = ("+" / "-") time-hour [[":"] time-minute]

time-zone = "Z" / time-numoffset

Furthermore, section 5.4 of the spec specifically recommends against including redundant information.

Java

The modern java.time classes built into Java use the standard ISO 8601 formats by default when parsing/generating strings. See Oracle Tutorial.

Parsing text input

With Z:

Instant instant = Instant.parse( "2019-01-23T12:34:56.123456789Z" ) ;

With +00:00:

OffsetDateTime odt = OffsetDateTime.parse( "2019-01-23T12:34:56.123456789+00:00" ) ;

Generating text output

To create a string with the Z, simply call Instant::toString.

String output = Instant.now().toString() ;  // Capture the current moment in UTC, then generate text representing that value in standard ISO 8601 using the `Z` offset-indicator.

2019-05-22T21:00:52.214709Z

To create a string with the 00:00, call OffsetDateTime::format. Generate text using a DateTimeFormatter with a formatting pattern you define.

DateTimeFormatter f = DateTimeFormatter.ofPattern( "uuuu-MM-dd'T'HH:mm:ss.SSSxxx" ) ;
String output = OffsetDateTime.now( ZoneOffset.UTC ).format( f ) ;

2019-05-22T21:00:52.319076+00:00

Truncating

You may want to truncate any microseconds or nanoseconds.

Instant
.now()
.truncatedTo( ChronoUnit.MILLIS )
.toString()

2019-05-22T21:11:28.970Z

…and…

OffsetDateTime
.now( ZoneOffset.UTC )
.truncatedTo( ChronoUnit.MILLIS )
.format( 
    DateTimeFormatter.ofPattern( "uuuu-MM-dd'T'HH:mm:ss.SSSxxx" )
)

2019-05-22T21:11:29.078+00:00

See this code running live at IdeOne.com.

Never use 'Z'

Never use 'Z' with the quote marks in your formatting pattern.

The pair of single-quote marks turns the Z into a string literal, mere decoration, rather than a meaningful pattern code.

The second argument of date is a UNIX timestamp, not a database timestamp string.

You need to convert your database timestamp with strtotime.

<?= date("c", strtotime($post[3])) ?>

I prefer using the dateutil library for timezone handling and generally solid date parsing. If you were to get an ISO 8601 string like: 2010-05-08T23:41:54.000Z you'd have a fun time parsing that with strptime, especially if you didn't know up front whether or not the timezone was included. pyiso8601 has a couple of issues (check their tracker) that I ran into during my usage and it hasn't been updated in a few years. dateutil, by contrast, has been active and worked for me:

from dateutil import parser
yourdate = parser.parse(datestring)