การเช็คคำ อยากรู้ว่าจะใช้ฟังชั้นอะไรช่วยในการตรวจสอบคำอ่ะคับ

<?php
$mystring = "wxyz";
$pos = strrpos($mystring, "b");
if ($pos === false) { 
    echo "not found<br>";
}
$mystring = "abcdef";
$pos = strrpos($mystring, "b");
if (is_bool($pos) && !$pos) {
    echo "not found<br>";
}
?>

<?php
$mystring = "abcdef";
$pos = strrpos($mystring, "a");
if (is_bool($pos) && !$pos) {
    echo "not found<br>";
}
?>

<?php
$mystring = " "."wxyz";	
$pos = strrpos($mystring, "w");
if ($pos) {
    echo "not found1<br>";
}else{
    echo "not found2<br>";
}
?>