PHP allows you to set dynamic variable names. This can be very useful when working with arrays and loops.

To use a variable in a variable declaration, wrap the declaration in curly brackets.

This is an example of a variable variable being used to build a form element in the context of a WordPress widget:

<?php

$options = ['mobile', 'landline']

foreach ($options as $option) {

  $fieldname = $option . '_text';

  ?>
  <label>
    Set the text for <?= $option; ?>
    <input
      type="text"
      name="<?= $this->get_field_name( $fieldname ); ?>"
      value="<?= ${$option . '_text'}; ?>" />
  </label><br>
  <?php

}
?>

In this example, during each loop the “value” attribute is set to a variable which is built on the fly. During the first iteration, the value attribute looks like value="<?= $mobile_text; ?>". This variable allows us to retrieve previously saved data that is represented by $mobile_text. On the next loop, the value will be set to $landline_text.

Resources

Variable variables for dummies PHP man page