If you're outputting the field with form_rest you'll have to set the label for the the field to false in the form builder with something like

$builder->add('first_name', 'text', array(
    'label' => false,
));

If you output the fields individually, you can omit the form_label for that field in the twig template, or set it to an empty string.

{{ form_label(form.first_name, '') }}

I did this recently! :) You'll want to create a new fields template, for form_row and one for form_widget. Then remove the form_label part, and add your placeholder.

http://symfony.com/doc/current/cookbook/form/form_customization.html

You can do it per field, or set it for all of them.

Or you can also skip the removing the form_label from the form_row template, and just do form_widget() where you're currently calling form_row()

You must render the form manually.

Here's an example:

<form id="form-message" action="{{ path('home') }}" method="post" {{ form_enctype(form) }}>
  {{ form_label(form.name) }}
  {% if form_errors(form.name) %}
    <div class="alert alert-error">
      {{ form_errors(form.name) }}
    </div>
  {% endif %}
  {{ form_widget(form.name) }}
  {{ form_row(form._token) }}
  <input type="submit" class="btn" value="Submit">
</form>

Related documentation

for other that come across this label-question: you could use form theme to override the form_row tag for every form you want. However I recommend to just set it invisible for page reader optimization. my example with bootstrap:

{% block form_row %}
    {% spaceless %}
            {{ form_label(form, null, {'label_attr': {'class': 'sr-only'}}) }}
            {{ form_errors(form) }}
            {{ form_widget(form) }}
    {% endspaceless %}
{% endblock form_row %}

don't forget to include your formtheme in config.yml and template.

Convert label to placeholder

{% use 'form_div_layout.html.twig' with widget_attributes as base_widget_attributes %}
{% block widget_attributes %}
    {% set attr = {'placeholder': label|trans({}, translation_domain)} %}
    {{- block('base_widget_attributes') -}}
{% endblock widget_attributes %}

Expanding on Léo's answer:

{% use 'form_div_layout.html.twig' %}
{% block widget_attributes %}
{% spaceless %}
    {% set attr = attr|merge({'placeholder': label}) %}
    {{ parent() }}
{% endspaceless %}
{% endblock widget_attributes %}

trans filter has been removed because it is included in the parent.

I know it's already answered, but might help somebody who is looking for a different solution for placeholders, if you don't want to change anything in your twig template:

$builder->add(
    'name',
    'text', 
     array(
        'attr' => array(
             'placeholder' => 'Your name',
        ),
        'label' => false,
     )
);

For those NOT using form_row, you can always add the placeholder as an attribute directly when adding the input to the builder. Like so:

$task = new Task();
$form = $this->createFormBuilder($task)
            ->add('first_name', 'text', array(
                      'required' => true,
                      'trim' => true,
                      'attr' => array('placeholder' => 'Lorem Ipsum')
        )->getForm();

Symfony 2.8 & above

1. Remove form_label

   {% block form_row %}
   <div>
       {{ form_errors(form) }}
       {{ form_widget(form) }}
   </div>
   {% endblock form_row %}
   

2. Add placeholder attribute

   {% block form_widget_simple %}
       {% set type = type|default('text') %}
       <input placeholder="{{ translation_domain is same as(false) ? label : label|trans({}, translation_domain) }}" type="{{ type }}" {{ block('widget_attributes') }} {% if value is not empty %}value="{{ value }}" {% endif %}/>
   {% endblock form_widget_simple %}
   

To sums it up:

• Titi's answer is the most simple ;

• Mick, Léo & Quolonel's answers are the most effective but are incomplete (for symfony > 2.6) :

If you use the label_format option in your *Type::configureOptions, their solution does not work. You need to add the content of the form_label block to handle all the label possibilities. The full & most effective answer (code used w/ symfony 3.3) :

1. Remove form_label

   {% block form_row %}
   <div>
       {{ form_errors(form) }}
       {{ form_widget(form) }}
   </div>
   {% endblock form_row %}
   

2. Edit the widget_attribute block

   {% block widget_attributes %}
       {% spaceless %}
           {% if label is not same as(false) -%}
               {% if label is empty -%}
                   {%- if label_format is not empty -%}
                       {% set label = label_format|replace({
                           '%name%': name,
                           '%id%': id,
                       }) %}
                   {%- else -%}
                       {% set label = name|humanize %}
                   {%- endif -%}
               {%- endif -%}
   
               {% set attr = attr|merge({'placeholder': label}) %}
           {%- endif -%}
   
           {{ parent() }}
       {% endspaceless %}
   {% endblock widget_attributes %}
   

Notes :

• Do not translate the labels into the widget_attributes block, otherwise they will appear as missing translations.

• The solution does not work for checkboxes or radio buttons, you'll want to add something like :

  {%- block checkbox_widget -%}
      {{ parent() }}
      {{- form_label(form) -}}
  {%- endblock checkbox_widget -%}
  

You can also copy the labels into the placeholder attribute before rendering the form:

$formView = $form->createView();

foreach($formView->getIterator() as $item) {
    /** @var $item FormView */
    if ($item->vars['label']) {
        $item->vars['attr']['placeholder'] =$item->vars['label'];
    }
}

Bootstrap Forms

In my case best is mix aswers of @Cethy and @Quolonel Questions

{% form_theme form _self %}
{% use 'bootstrap_4_layout.html.twig' %}

{% block widget_attributes %}          {#   set placeholder   #}
    {% spaceless %}
        {% set attr = attr|merge({'placeholder': label}) %}
        {{ parent() }}
    {% endspaceless %}
{% endblock widget_attributes %}

{% block form_row %}                   {#   remove label   #}
    <div class="form-group">
        {{ form_errors(form) }}
        {{ form_widget(form) }}
    </div>
{% endblock form_row %}

It looks the following

It works with translations