Execute command multiple times with curly brackets arguments list

That's called Brace Expansion, which expands to a space-separated list of the given strings.

So touch {a,b,c} would be equivalent to

touch a b c

While touch {a,b,c}x would be equivalent to:

touch ax bx cx

You pear command would essentially be run as:

pear channel-discover pear.phpunit.de pear.symfony-project.com

which may not be what you expected. If you want the command to be run once for each string, use a for loop (which answers your 2nd question), or use a combination of brace expansion and xargs.

The problem is that contrary to your expectation, the brace expansion of

touch {a,b,c}

is equivalent to

touch a b c   # NOT 3 separate invocations.

(Use echo {a,b,c} to verify). It appears that pear channel-discover does not accept two args. You will probably see the same error with

pear channel-discover pear.phpunit.de pear.symfony-project.com

Well, you have two options:

for i in "pear.phpunit.de" "pear.symfony-project.com"do  pear channel-discover "$i"done

Or a one-liner (but calling xargs instead of using bash internals):

echo "pear.phpunit.de" "pear.symfony-project.com" | xargs -n1 pear channel-discover

The former is certainly easier to read by the human, the time efficiency will be basically the same.