Search array keys with wildcard

This one assumes number needs to be followed by an actual number (edit: or nothing at all), adjust the regular expression as necessary. For example, anything starting with 'number', you could use /^number/.

if(count(preg_grep('/^number[\d]*/', array_keys($arr))) > 0)
{
   return true;
}
else
{
   return false;
}

Use regular expression.

foreach ($arr as $key => $value) {
  // NOTE: check for the right format of the regular expression 
  if (preg_match("/^number([0-9]*)$", $key)) {
    echo "A match was found.";
  } else {
    echo "A match was not found.";
  }
}