Re: pg_dump's "--exclude-table" and "--exclude-table-data" options are ignored and/or cause the dump to fail entirely unless both the schema and table name use 1950s-era identifiers. - Mailing list pgsql-bugs

Not sure about encoding dynamics but your issue is likely with writing shell commands.  Instead of trying to write a full pg_dump command I suggest trying to set the value of an environment variable to the double-quoted value you want (checking it with echo) and then just supplying that variable with the pg_dump command.

This sounded promising at first, and I have been forced to do something similar before to provide the password with "PGPASSWORD" as it wouldn't let me enter this on the command line as an argument, but unless I misunderstand you, my:

--exclude-table-data=%TEMPTESTFORPG%

... isn't turned into the value for "TEMPTESTFORPG",

which I set with this in PHP:

putenv('TEMPTESTFORPG="Schema name"."Table name"');

I also tried with:

--exclude-table-data=' . getenv('TEMPTESTFORPG')

.... but it just has the same stupid result with pg_dump either dumping the nonsensical errors or executing the command but ignoring my "exclude rule" (and thus dumping the table's data).