<?php
$arraystring = ['test', 'cat', 'meow'];
$letter = "ca";
checkEarraystring($arraystring,$letter);
function checkEarraystring ($arraystring,$letter)
{
    $string = array();
    foreach ($arraystring as $value)
    {
        if (strpos($value,$letter) !== false)
        {

            $string[] = $value;
        }
        else {
            echo  "eror";
        }
    }
    return $string;
}
var_dump(checkEarraystring($arraystring,$letter));