How can I extract the text of HTML5 Constraint validation using PHP + Selenium

Question

When incorrect value is entered in textbox next hint is appeared:

I have tried approach like for Java + Selenium . But it did not work for PHP. I did not get validation error message:

HTML:

<form action="/login" method="post" class="form-horizontal"> <input type="hidden"> 
    <div class="form-group form-group-withLabel"> 
        <input type="email" name="_username" id="inputEmail" required="" autofocus="" autocomplete="off"> 
        <label for="inputEmail" class="inner-label">Email address</label> 
    </div> 
    <div class="form-group form-group-withLabel"> 
        <input type="password" name="_password" id="inputPassword" placeholder="Password" required=""> 
        <label class="inner-label" for="inputPassword">Password</label> 
    </div> 
</form>

System.out.println(new WebDriverWait(driver, 20).until(ExpectedConditions.elementToBeClickable(By.xpath("//input[@id='inputEmail' and @name='_username']"))).getAttribute("validationMessage")); 

$this->client->wait()->until(WebDriverExpectedCondition::elementToBeClickable($by));             
$attr = $this->client->findElement($by)->getAttribute('validationMessage');