collision rate of base64 of md5 split at 7th letter

test.php

#!php -d memory_limit=10G test.php
<?php

$a = [];
$total = 0;
$collision = 0;

for ($i = 0; $i < 100000000; $i++) {
  $total += 1;
  for ($j = 0; $j < 2; $j++) {
    $h = substr(base64_encode(md5($j . $i, true)), 0, 7);
    if (isset($a[$h])) {
      $collision += 1;
      continue;
    } else {
      $a[$h] = $i;
      if ($j > 0) {
        echo $i, "\t", $j, "\t", $total, "\t", $collision, "\t", ($collision / $total), PHP_EOL;
      }
      break;
    }
  }
}

//var_export($a);