まだまだ、バグがありますが、
とりあえず動くものをつくってみました。
<!DOCTYPE html> <html lang="ja"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>電卓</title> </head> <body> <form action="dentaku-test1.php" method="post"> <?php if (isset($_POST ["iko-ru"])) { if($_POST ["keisan"] == "÷"){ $_display = $_POST ["_sahen"] / $_POST ["_2keta"]; echo '<input type="text" value=' .$_display.' name="_2keta">'; } if($_POST ["keisan"] == "×"){ $_display = $_POST ["_sahen"] * $_POST ["_2keta"]; echo '<input type="text" value=' .$_display.' name="_2keta">'; } if($_POST ["keisan"] == "-"){ $_display = $_POST ["_sahen"] - $_POST ["_2keta"]; echo '<input type="text" value=' .$_display.' name="_2keta">'; } if($_POST ["keisan"] == "+"){ $_display = $_POST ["_sahen"] + $_POST ["_2keta"]; echo '<input type="text" value=' .$_display.' name="_2keta">'; } } else{ if (isset($_POST ["keisan"])) { if (isset($_POST ["_sahen"])) { echo '<div id="sahen"><input type="text" value=' .$_POST ["_sahen"].' name="_sahen"></div>'; echo '<div id="keisan"><input type="text" value=' .$_POST ["keisan"].' name="keisan"></div>'; if (isset($_POST ["_2keta"])){ $_display = $_POST ["_2keta"] . $_POST ["main"] ; echo '<input type="text" value=' .$_display.' name="_2keta">'; } else {echo '<input type="text" value="' .$_POST ["main"].'" name="_2keta">'; } } else{ echo '<div id="keisan"><input type="text" value=' .$_POST ["_2keta"].' name="_sahen"></div>'; echo '<input type="text" value=' .$_POST ["keisan"].' name="keisan">'; } } else{ if (isset($_POST ["main"])) { if (isset($_POST ["_2keta"])){ $_display = $_POST ["_2keta"] . $_POST ["main"] ; echo '<input type="text" value=' .$_display.' name="_2keta">'; //echo $_2keta; echo $_POST ["main"];// } else { echo '<input type="text" value="' .$_POST ["main"].'" name="_2keta">'; } } else{ echo '<input type="text" value="0" name="display">'; } } } ?> <br> <input type="submit" value="CE" name="ce"> <input type="submit" value="C" name="c"> <input type="submit" value="BS" name="bs"> <input type="submit" value="÷" name="keisan"> <br> <input type="submit" value="7" name="main"> <input type="submit" value="8" name="main"> <input type="submit" value="9" name="main"> <input type="submit" value="×" name="keisan"> <br> <input type="submit" value="4" name="main"> <input type="submit" value="5" name="main"> <input type="submit" value="6" name="main"> <input type="submit" value="-" name="keisan"> <br> <input type="submit" value="1" name="main"> <input type="submit" value="2" name="main"> <input type="submit" value="3" name="main"> <input type="submit" value="+" name="keisan"> <br> <input type="submit" value="±" name="pm"> <input type="submit" value="0" name="main"> <input type="submit" value="." name="main"> <input type="submit" value="=" name="iko-ru"> </form> </body> </html>