Php和HTML表单 - 输入字段中的随机值

I write a conversion from celsius to fahrenheit and vice versa. The problem is that in the form fields now random values are displayed to me. How to convert this code so that after entering the value, the converted value in the second field appears in the first field? Is it possible to use only php at all?

if(isset($_POST['fah'])){
    $cel=($_POST['fah']+32)/1.8;
}
if(isset($_POST['cel'])){
    $fah=($_POST['cel']-32)*1.8;
}

?>
<html>
<body>


<form method="post" action="calc.php">
Fahrenheit: <input id="inFah" type="text" placeholder="Fahrenheit" value="<?php echo $fah; ?>" name="fah">
Celcius: <input id="inCel" type="text" placeholder="Celcius" value="<?php echo $cel; ?>" name="cel">
<input type="submit" value="Calc">

</form>

I want the value entered in the first field to be shown in the second transformed.

You can do all php if you post back to itself. If the page with the input is calc.php Add an else to set the value to empty string.

if(isset($_POST['fah'])){
    $cel=($_POST['fah']+32)/1.8;
} else {
    $cel = '';
}
if(isset($_POST['cel'])){
    $fah=($_POST['cel']-32)*1.8;
} else {
    $fah = '';
}

Try something like this

$cel="";
$fah="";

if(isset($_POST['fah']) && !empty($_POST['fah'])){
    $cel=($_POST['fah']+32)/1.8;
}
if(isset($_POST['cel']) && !empty($_POST['cel'])){
    $fah=($_POST['cel']-32)*1.8;
}

You can try this example:

$celValue = $_POST['cel'];
$fahValue = $_POST['fah'];

if( ! empty($fahValue)){
    $celValue = fahrenheitToCelcius($_POST['fah']);
}
if( ! empty($celValue)){
    $fahValue = celciusToFahrenheit($_POST['cel']);
}

function celciusToFahrenheit($cel) {
    return ($cel - 32) * 1.8;
}

function fahrenheitToCelcius($fah) {
    return ($fah + 32) / 1.8;
}

?>
<html>
<body>


<form method="post" action="calc.php">
Fahrenheit: <input id="inFah" type="text" placeholder="Fahrenheit" value="<?php echo $celValue; ?>" name="fah">
Celcius: <input id="inCel" type="text" placeholder="Celcius" value="<?php echo $fahValue; ?>" name="cel">
<input type="submit" value="Calc">

</form>

Since both the variables give isset() true so we can have something like this

if(isset($_POST['fah']) && isset($_POST['cel'])) {
    //if cel is not empty
    if(!empty($_POST['cel'])) {
       $cel = $_POST['cel'];
       $fah=($cel-32)*1.8;
    } else if(!empty($_POST['fah']){
       $fah = $_POST['fah'];
       $cel = ($fah+32)/1.8;
    }
}

?>
<html>
<body>


<form method="post" action="calc.php">
Fahrenheit: <input id="inFah" type="text" placeholder="Fahrenheit" value="<?php echo $fah; ?>" name="fah">
Celcius: <input id="inCel" type="text" placeholder="Celcius" value="<?php echo $cel; ?>" name="cel">
<input type="submit" value="Calc">

</form>

You just missing set default values that will overwrite if other post value exist, and else if to load only one parameter so other will be empty

    <?php
    $cel = "";
    $fah = "";

    if(isset($_POST['fah'])){
        $cel=($_POST['fah']+32)/1.8;
    }elseif(isset($_POST['cel'])){
        $fah=($_POST['cel']-32)*1.8;
    }

    ?>
    <html>
    <body>


    <form method="post" action="calc.php">
    Fahrenheit: <input id="inFah" type="text" placeholder="Fahrenheit" value="<?php echo $fah; ?>" name="fah">
    Celcius: <input id="inCel" type="text" placeholder="Celcius" value="<?php echo $cel; ?>" name="cel">
    <input type="submit" value="Calc">

    </form>