PHP
<?php
header("Content-type: text/html; charset=utf-8");
$data1 = "PHP:ピーエッチピー";
$data2 = "ＰＨＰ：ピーエッチピー";
$data3 = "PHP:Hypertext Preprocessor";
$data4 = "";
//半角全角混在
echo $data1 . "<br>";
echo "→" . mb_strlen($data1,"utf-8") . "<br>";
echo "→" . strlen($data1) . "<br><br>";
//全角のみの文字列の長さ
echo $data2 . "<br>";
echo "→" . mb_strlen($data2,"utf-8") . "<br>";
echo "→" . strlen($data2) . "<br><br>";
//半角のみの文字列の長さ
echo $data3 . "<br>";
echo "→" . mb_strlen($data3,"utf-8") . "<br>";
echo "→" . strlen($data3) . "<br><br>";
//空の文字列の長さ
echo $data4 . "<br>";
echo "→" . mb_strlen($data4,"utf-8") . "<br>";
echo "→" . strlen($data4) . "<br><br>";
?>