Simple answer
$p = 1234567;
$p = sprintf("%08d",$p);
I'm not sure how to interpret the comment saying "It will never be more than 8 digits" and if it's referring to the input or the output. If it refers to the output you would have to have an additional substr() call to clip the string.
To clip the first 8 digits
$p = substr(sprintf('%08d', $p),0,8);
To clip the last 8 digits
$p = substr(sprintf('%08d', $p),-8,8);