Concatenate values of n arrays in php

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You can put all word arrays into one array and use a recursive function like this:

function concat(array $array) {
    $current = array_shift($array);
    if(count($array) > 0) {
        $results = array();
        $temp = concat($array);
        foreach($current as $word) {
          foreach($temp as $value) {
            $results[] =  $word . ' ' . $value;
          }
        }
        return $results;           
    }
    else {
       return $current;
    }
}

$a = array(array('dog', 'cat'), array('food', 'tooth'), array('car', 'bike'));

print_r(concat($a));

Which returns:

Array
(
    [0] => dog food car
    [1] => dog food bike
    [2] => dog tooth car
    [3] => dog tooth bike
    [4] => cat food car
    [5] => cat food bike
    [6] => cat tooth car
    [7] => cat tooth bike
)

But I guess this behaves badly for large arrays as the output array will be very big.

To get around this, you can output the combinations directly, using a similar approach:

function concat(array $array, $concat="") {
    $current = array_shift($array);

    $current_strings = array();

    foreach($current as $word) {
            $current_strings[] = $concat . ' ' . $word;
    }

    if(count($array) > 0) {
        foreach($current_strings as $string) {
            concat($array, $string);
        }       
    }
    else {
      foreach($current_strings as $string) {
          echo $string . PHP_EOL;
      }   
    }
}

concat(array(array('dog', 'cat'), array('food', 'tooth'), array('car', 'bike')));

Which gives:

dog food car
dog food bike
dog tooth car
dog tooth bike
cat food car
cat food bike
cat tooth car
cat tooth bike

With this approach it is also easy to get the “sub-concatinations”. Just insert echo $string . PHP_EOL; before concat($array, $string); and the output is:

 dog
 dog food
 dog food car
 dog food bike
 dog tooth
 dog tooth car
 dog tooth bike
 cat
 cat food
 cat food car
 cat food bike
 cat tooth
 cat tooth car
 cat tooth bike

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