Say you have the following validation message in the custom array of your language file - resources/lang/xx/validation.php - (or another location depending on your use case).
<?php
return [
// ...
'custom' => [
'email' => [
'supported_email_provider' => ':provider not supported at this time.',
],
],
// ...
];
supported_email_provider is a custom validation rule here and :provider is a custom placeholder within the validation message.
Properly passing in a value for :provider has got me searching the web the last couple hours, fortunately, I found this Stack Overflow answer that provides a clean solution to this, check it out!