Create fixed length non-repeating permutation of larger set

Question

I know this topic is much discussed but I can\'t seem to find any implementation that fits my needs.

I have the following set of characters:

a b c

Answer 1

$strings = get_perm( range('a', 'h'), 4 );

function get_perm( $a, $c, $step = 0, $ch = array(), $result = array() ){
    if( $c == 1 ){ //if we have last symbol in chain
        for( $k = 0; $k < count( $a ); $k++ ){
            if( @in_array( $k, $ch ) ) continue; // if $k exist in array we already have such symbol in string
            $tmp = '';

            foreach( $ch as $c ) $tmp .= $a[$c]; // concat chain of previous symbols
            $result[] = $tmp . $a[$k]; // and adding current + saving to our array to return
        }
    }else{
        for( $i = 0; $i < count( $a ); $i++ ){
            if( @in_array( $i, $ch ) ) continue;
            $ch[$step] = $i; // saving current symbol for 2 things: check if that this symbol don't duplicate later and to know what symbols and in what order need to be saved
            get_perm( $a, $c-1, $step+1, $ch, &$result ); 
            // recursion, 
            // decrementing amount of symbols left to create string, 
            // incrementing step to correctly save array or already used symbols, 
            // $ch - array of already used symbols, 
            // &$result - pointer to result array
        }
    }

    return $result;
}

NOTICE

a-h with 6 symbols = 20k values in array
a-z with 4 symbols = 358799 values in array
So a-z with 10 symbols will die for sure =) It will require too much memory.
You need to try to save output to file or database if you would need big amount of values. Or extend memory limit to php but not sure if this is best way.