<?php
function t ( $val )
{
    echo preg_replace("/[^0-9a-zA-Z ]/", "", $val) . "\n";
}

t( "I am a string" ); // "I am a string"
t( "How are you?" ); // "How are you"
t( "#53-Jeff" ); // "53Jeff"

I am a string
How are you
53Jeff