<?php
$email = 'johndoe@example.com';
$username = 'johndoe';
$to_replace = array( "{username}", "{email}" );
$replace_with = array( $username, $email );
 
$key_value = array(
  'site' => 'abc.com',
  'blog' => 'blog.com',
  'roll' => 42
);
 
$to_replace = array_merge($to_replace, array_keys($key_value));
$replace_with = array_merge($replace_with, array_values($key_value));
 
array_walk($to_replace, function(&$value, $key) { $value = '{'.trim($value, '{}').'}';});
 
$message = 'This is a {username} speaking, my email is {email}, and my site is {site} with roll {roll}';
$message = str_replace( $to_replace, $replace_with, $message );
 
var_dump($message);

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