Sure! Basically if you take where we started : $u = ('0' x (6-length($u))).$u; we can break this up into multiple statements:

$length_of_u = length($u); # how long is this string?
$how_many_characters_short = 6-$length_of_u;  # how far are we from ha
+ving 6 characters?

$padding = '0' x $how_many_characters_short; # this will give us as ma
+ny 0's as $how_many_characters_short is long (e.g. if it is 4 - it gi
+ves us '0000')

$u = $padding.$u; # concatenate them together... finished!
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HTH!