configuration.php

// GLOBAL $link; <== remove this
$link = mysqli_connect($DB["host"], $DB["user"], $DB["pass"],$DB['dbName']) or die(mysqli_error($link));

yourfile.php

include 'configuration.php';

function checkInput($inputText){
    global $link;
    $outputText=mysqli_real_escape_string($link,trim($inputText));
    return $outputText;
}