Justify string algorithm [closed]

Here’s what I came up with. I added the optional $char parameter so you can see what it’s outputting – Of course you can pull it inside the function so the prototype matches the requirement.

function justify($str_in, $desired_length, $char="_") {

    // Some common vars and simple error checking / sanitation
    $return = '';
    $str_in = trim( $str_in);
    $desired_length = intval( $desired_length);

    // If we've got invalid input, we're done
    if( $desired_length <= 0)
        return $str_in;

    // If the input string is greater than the length, we need to truncate it WITHOUT splitting words
    if( strlen( $str_in) > $desired_length) {
        $str = wordwrap($str_in, $desired_length);
        $str = explode("\n", $str);
        $str_in = $str[0];
    }

    $words = explode( ' ', $str_in);
    $num_words = count( $words);

    // If there's only one word, it's a simple edge case
    if( $num_words == 1) {
        $length = ($desired_length - strlen( $words[0])) / 2;
        $return .= str_repeat( $char, floor( $length)) . $words[0] . str_repeat( $char, ceil( $length));
    } else {
        $word_length = strlen( implode( '', $words));

        // Calculate the number of spaces to distribute over the words
        $num_words--; // We're going to eliminate the last word
        $spaces = floor( ($desired_length - $word_length) / $num_words);
        $remainder = $desired_length - $word_length - ($num_words * $spaces);

        $last = array_pop( $words);
        foreach( $words as $word) {
            // If we didn't get an even number of spaces to distribute, just tack it on to the front
            $spaces_to_add = $spaces;
            if( $remainder > 0) {
                $spaces_to_add++;
                $remainder--;
            }

            $return .= $word . str_repeat( $char, $spaces_to_add);
        }
        $return .= $last;
    }
    return $return;
}

And the test cases:

$inputs = array( 
    'hello world there ok then',
    'hello',
    'ok then',
    'this string is almost certainly longer than 48 I think',
    'two words',
    'three ok words',
    '1 2 3 4 5 6 7 8 9'
);

foreach( $inputs as $x) {
    $ret = justify( $x, 48);
    echo 'Inp: ' . $x . " - strlen(" . strlen( $x) .  ")\n";
    echo 'Out: ' . $ret . " - strlen(" . strlen( $ret) .  ")\n\n";
}

And the output:

Inp: hello world there ok then - strlen(25)
Out: hello_______world_______there_______ok______then - strlen(48)

Inp: hello - strlen(5)
Out: _____________________hello______________________ - strlen(48)

Inp: ok then - strlen(7)
Out: ok__________________________________________then - strlen(48)

Inp: this string is almost certainly longer than 48 I think - strlen(54)
Out: this_string_is_almost_certainly_longer_than_48_I - strlen(48)

Inp: two words - strlen(9)
Out: two________________________________________words - strlen(48)

Inp: three ok words - strlen(14)
Out: three__________________ok__________________words - strlen(48)

Inp: 1 2 3 4 5 6 7 8 9 - strlen(17)
Out: 1_____2_____3_____4_____5_____6_____7_____8____9 - strlen(48)

And a demo!

Edit: Cleaned up the code, and it still works :).