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# Mod_rewrite is great at manipulating HTTP requests.
# Using it to set and read temp env vars is a helpful technique.
#
# This example walks through fixing a query string:
# Extract good query params, discard unwanted ones, reorder good ones, append one new one.
#
# Before: /before?badparam=here&baz=w00t&foo=1&bar=good&mood=bad
# After: /after?foo=1&bar=good&baz=w00t&mood=happy
#
# Storing parts of the request (or anything you want to insert into it) in ENV VARs is convenient.
# Note the special RewriteRule target of "-" which means "no redirect; simply apply side effects"
# This lets you manipulate the request at will over multiple steps.
#
# In a RewriteRule, set custom temp ENV VARs via [E=NAME:value]
# Note it's also possible to set multiple env vars
# like [E=VAR_ONE:hi,E=VAR_TWO:bye]
#
# You can read these values using %{ENV:VAR_NAME}e <- little "e" is not a typo
#
# Tangent:
# Note you can also read these env vars the same way, if you set them via SetEnvIf[NoCase]
# (It won't work to use SetEnv, which runs too early for mod_rewrite to pair with it.)
#
# Regex details:
# (?:) syntax means "match but don't store group in %1 backreference"
#      so (?:^|&) is simply the ^ beginning or an & delimiter
#      (the only 2 possibilities for the start of a qs param)
# ([^&]+) means 1 or more chars that are not an & delimiter

RewriteCond %{QUERY_STRING} (?:^|&)foo=([^&]+)
RewriteRule ^/before - [E=FOO_VAL:%1]

RewriteCond %{QUERY_STRING} (?:^|&)bar=([^&]+)
RewriteRule ^/before - [E=BAR_VAL:%1]

RewriteCond %{QUERY_STRING} (?:^|&)baz=([^&]+)
RewriteRule ^/before - [E=BAZ_VAL:%1]

RewriteRule ^/before /after?foo=%{FOO_VAL}e&bar=%{BAR_VAL}e&baz=%{BAZ_VAL}e&mood=happy [R=301,L]

# That's it! No PHP needed! :)