if(isset($_POST['revisionCad'])){
    $revisionCad = $_POST['revisionCad'];
}

if(empty($revisionCad)){
    erDisplay('Please select a revision for Cadgui');
}

var_dump(false == 0); // Outputs "true"
var_dump(empty(0));   // Outputs "true"

if (isset($_POST['revisionCad']) && $_POST['revisionCad'] >= 0) {
    $revisionCad = $_POST['revisionCad'];
} else {
    erDisplay('Please select a revision for Cadgui');
}

<form method= "POST">
<!-- <input type="number" class="form-control" placeholder="Revision" name="revisionCad" id="revisionCad" min="0"> -->

/** Assign default value or set it automatically in your code.**/
<input type="number" class="form-control" placeholder="Revision" name="revisionCad" id="revisionCad" min="0" value=0>

<button name="submit" class="btn btn-primary" id="submit" tabindex="5" value="Save" type="submit" style="width:200px;">Create/Change Revision</button>
</form>

if(isset($_POST['revisionCad'])){
  $revisionCad = $_POST['revisionCad'];
}


if(empty($revisionCad)){
    erDisplay('Please select a revision for Cadgui');
    }

// You can set a default value of 0 in the code or leave it at null;
    $revisionCad = (isset($_POST['revisionCad']) && !empty( $_POST['revisionCad'] ) || $_POST['revisionCad'] >=0 ) ?
    trim($_POST['revisionCad']) : null;


/* Check if data is set to default null by your script,
 * Display error, since no data was received
 */
if(is_null($revisionCad))
{
  erDisplay('Please select a revision for Cadgui');
}

// if no error, continue with the rest of the codes ...

if(isset($revisionCad) && $revisionCad >=0){
    erDisplay('Please select a revision for Cadgui');
}

<input type="number" class="form-control" placeholder="Revision" name="revisionCad" id="revisionCad" min="0" value="0">