<?php

$valid_input = $_POST['input_text'];

if (isset($_POST['input_char'])){
    $locations = array();
    for ($i=0; $i < strlen($valid_input); $i++) { 
        if ($_POST['input_char'] == $valid_input[i]){
            array_push($locations, $i);
        }
    }

    echo $locations[0];
}