Use Predis EVAL with a Redis Lua script to decrement a key, delete the key if the new value is zero, and then return the new value atomically.

predis-decrement-delete.php

<?php

$lua = <<<LUA
    local c = redis.call('DECR', KEYS[1])
    if c == 0 then 
        redis.call('DEL', KEYS[1]) 
    end
    return c
LUA;

// Argument 1 is the Lua script.
// Argument 2 is the number of keys being passed to the script (always 1 for this script).
// Argument 3 is the key to decrement/delete.
$count = $predisClient->eval($lua, 1, 'foo-bar-key');

// An alternative would be to use a WATCH + MULTI + EXEC transaction, but the Redis community seems to prefer Lua.