Attach Web Event Receivers using Powershell

# Defintion of function to attach web event receivers
# if they are not already added
Function AddWebEventReceivers($webER)
{
  $assembly = "*fully_qualified_assembly_name*"
  $class = "*class_name_including_namespace*"
  # To Check fully qualified assembly name Click Here
  # Only attach receivers if there aren't already added
  # You can make the check more specific by checking the Type
  # property as well if required
  $existingER = $webER.EventReceivers | Where { $_.Class -eq $class }
  if($existingER -eq $null -or $existingER.length -eq 0)
  {
    $webER.EventReceivers.Add("WebDeleting", $assembly, $class)
  }
}

# Iterate all webs and attach the web event receivers to
$web = Get-SPWeb "*SubsiteUrl*"
$subwebs = $web.GetSubwebsForCurrentUser()
foreach($web in $subwebs)
{
  try
  {
    AddWebEventReceivers($web)
    Write-Host "Event Receiver attached to web: $web"
  }
  catch [System.Exception]
  {
    $errorMessage = $Error[0]
    Write-Host "Failed: $errorMessage" -NoNewline -F Red
  }
  finally
  {
    $web.Dispose()
  }
}
$web.Dispose()