<?php
$arraystring = ['test', 'cat', 'meow'];
checkEarraystring($arraystring,$letter = "e");
function checkEarraystring ($arraystring,$letter = "e"){
    $string = array();
    foreach ($arraystring as $value) {
        $string[] = $value >= $letter;
    }
    return $string;
}
var_dump(checkEarraystring($arraystring));