Regular expression with variable

I want to replace the string [USER_NAME] with an appropriate value.

preg_replace( "/[USER_NAME]/", $user_name, $text )

works nice but how do I do it if the string to replace is in a variable?

$variable = "[USER_NAME]"; //This row doesnt exist - I get the value from a file.

preg_replace( $variable, $value, $text );

maybe simple but its way over my head :-)

try
preg_replace("/" . escapeshellcmd($to_replace) . "/", $replacement, $string);

Wonderful! Thanks a lot! I owe u a cake or something 🙂