<?php

// Правильные: 
$correctNumbers = [ 
  '84951234567', 
  '+74951234567',
  '8-495-1-234-567', 
  ' 8 (8122) 56-56-56',
  '8-911-1234567',
  '8 (911) 12 345 67', 
  '8-911 12 345 67',
  '8 (911) - 123 - 45 - 67',
  '+ 7 999 123 4567', 
  '8 ( 999 ) 1234567',
  '8 999 123 4567'
];

$results = preg_replace(['/[\s()-]/', '/^\+\s?7/'], ['', '8'], $correctNumbers);
print_r($results);