php - HTML form submission with empty variables (dynamically write/not write to file)

I have a HTML form that writes user entered data to a file and puts it into a specific directory using a PHP script which is working well.

Here is my HTML page:

<form id="form" action="write.php" class="u-clearfix u-form-spacing-25 u-form-vertical u-inner-form" name="form"  method="POST" style="padding: 10px;" redirect="true" redirect-address="write.php">
            <div class="u-form-group u-form-name u-form-partition-factor-2">
              <label for="field1" class="u-custom-font u-heading-font u-label u-text-body-alt-color u-label-1">field1</label>
              <input id="field1" type="text" placeholder="field1" name="field1" minlength="7" maxlength="15" pattern="^((\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.){3}(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])$" input required class="u-border-1 u-border-custom-color-1 u-custom-font u-heading-font u-input u-input-rectangle u-radius-50 u-white u-input-1" required="required">
            </div>
            <div class="u-form-email u-form-group u-form-partition-factor-2">
              <label for="field2" class="u-custom-font u-heading-font u-label u-text-body-alt-color u-label-2">field2</label>
              <input id="field2" type="number" placeholder="field2" name="field2" class="u-border-1 u-border-custom-color-1 u-custom-font u-heading-font u-input u-input-rectangle u-radius-50 u-white u-input-2" required="required">
            </div>
            <div class="u-form-group u-form-select u-form-group-3">
              <label for="field3" class="u-custom-font u-heading-font u-label u-text-body-alt-color u-label-3">field3</label>
              <div class="u-form-select-wrapper">
                <select id="field3" name="field3" class="u-border-1 u-border-custom-color-1 u-custom-font u-heading-font u-input u-input-rectangle u-radius-50 u-white u-input-3" required="required">
                  <option value="option1">option1</option>
                  <option value="option2">option2</option>
                  <option value="option3">option3</option>
                  <option value="option4">option4</option>
                </select>
                <svg xmlns="http://www.w3.org/2000/svg" width="14" height="12" version="1" class="u-caret"><path fill="currentColor" d="M4 8L0 4h8z"></path></svg>
              </div>
            </div>
            <div class="u-form-group u-form-partition-factor-2 u-form-select u-form-group-4">
              <label for="field4" class="u-custom-font u-heading-font u-label u-text-body-alt-color u-label-4">field4</label>
              <div class="u-form-select-wrapper">
                <select id="field4" name="field4" class="u-border-1 u-border-custom-color-1 u-custom-font u-heading-font u-input u-input-rectangle u-radius-50 u-white u-input-4">
                  <option value="1">100 (Default)</option>
                  <option value="10">1000</option>
                  <option value="100">10,000</option>
                  <option value="1000">100,000</option>
                </select>
                <svg xmlns="http://www.w3.org/2000/svg" width="14" height="12" version="1" class="u-caret"><path fill="currentColor" d="M4 8L0 4h8z"></path></svg>
              </div>
            </div>
            <div class="u-form-group u-form-partition-factor-2 u-form-group-5">
              <label for="field5" class="u-custom-font u-heading-font u-label u-text-body-alt-color u-label-5">field5</label>
              <input type="number" placeholder="field5" id="field5" name="field5" class="u-border-1 u-border-custom-color-1 u-custom-font u-heading-font u-input u-input-rectangle u-radius-50 u-white u-input-5" required="required">
            </div>
            <div class="u-align-left u-form-group u-form-submit">
              <a href="#" class="u-btn u-btn-round u-btn-submit u-button-style u-custom-color-2 u-custom-font u-heading-font u-radius-50 u-btn-1">Submit</a>
              <input type="submit" name="submit" value="submit" class="u-form-control-hidden">
            </div>
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Here is my PHP script:

<?php
if ( isset( $_POST['field1'] ) && isset($_POST['field2']) && isset($_POST['field3']) && isset($_POST['field4']) && isset($_POST['field5']) ) {
    $path = '/usr/local/data/' . trim($_POST['field1']) . ',' . trim($_POST['field2']) . ',' . trim($_POST['field3']) . '.conf';
    if ( $fh = fopen($path,"a+") ) {
        $string = 'field1='.$_POST['field1']. "\n". 'field2='.$_POST['field2']. "\n". 'field3='.$_POST['field3']. "\n". 'field4='.$_POST['field4']. "\n". 'field5='.$_POST['field5'];
        if ( fwrite($fh,$string) ) {
        } else {

        }
        fclose($fh);
    } else {

    }

} else {

}
?>
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I have slowly been adding more inputs into the HTML form for more config options for the user however I am having an issue with creating non-required inputs. For example all the fields in the current form have to be entered to submit the form however I want to add a new field which is not mandatory yet if this is completed the PHP script will write it to the .conf file and if it isn't completed not write anything.

Currently it write the input to a .conf file like so as all of the fields have to be completed:

field1="field1 input"
field2="field2 input"
field3="field3 input"
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I would like it so if I add say field6 to the HTML which sometimes may be completed by the user and sometimes not, the PHP script writes the .conf file like the below.

If field6 is populated on submit:

field1="field1 input"
field2="field2 input"
field3="field3 input"
field6="field6 input"
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If field6 is not populated on submit:

field1="field1 input"
field2="field2 input"
field3="field3 input"
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I can do the required config on the HTML however I am not sure how this would work on the PHP side. Could someone please point me in the right direction or help me out with this? Thanks in advance!

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