Whats the cleanest way to convert a 5-7 digit number into xxx/xxx/xxx format in php?

function formatMyNumber($num)
{
    return sprintf('%03d/%03d/%03d',
                   $num / 1000000,
                  ($num / 1000) % 1000,
                   $num % 1000);
}

$padded = str_pad($number, 9, '0', STR_PAD_LEFT);
$split = str_split($padded, 3);
$formatted = implode('/', $split);

function FormatWithSlashes($number)
{
    return substr(preg_replace('/(d{3})?(d{3})?(d{3})$/', '$1/$2/$3',
            '0000' . $number),
            -11, 11);
}

$numbers = Array(12345, 345678, 9876543);
foreach ($numbers as $val)
{
    $r = FormatWithSlashes($val);
    echo "<p>$r</p>";
}

function fmt($x) {
    return substr(number_format($x+1000000000, 0, ".", "/"), 2);
}

function FormatWithSlashes($number)
{
    return preg_replace('/^0*(d{3})(d{3})(d{3})$/', '$1/$2/$3',
            '0000' . $number);
}

substr('0000' . $number, -9, 9);

use 5.010;

sub myformat(_;$){

  # prepend with zeros
  my $_ = 0 x ( 9-length($_[0]) ) . $_[0];

  my $join = $_[1] // '/'; # using the 'defined or' operator `//`

  # m// in a list context returns ($1,$2,$3,...)
  join $join, m/ ^ (d{3}) (d{3}) (d{3}) $ /x;

}

$_ = 11111;
say myformat;
say myformat(2222);
say myformat(33333,';');
say $_;

Perl 5.8

sub myformat(;$$){
  local $_ = @_ ? $_[0] : $_

  # prepend with zeros
  $_ = 0 x ( 9-length($_) ) . $_;

  my $join = defined($_[1]) ? $_[1] :'/';

  # m// in a list context returns ($1,$2,$3,...)
  join $join, m/ ^ (d{3}) (d{3}) (d{3}) $ /x;

}

def convert(num): #num is an integer
    a = str(num)
    s = "0"*(9-len(a)) + a
    return "%s/%s/%s" % (s[:3], s[3:6], s[6:9])