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.flex-container {
	display: -webkit-flex;
	display: flex;
	-webkit-flex-flow: row wrap;
	flex-flow: row wrap;
	-webkit-justify-content: center;
	justify-content:center;  
}

.flex-item {
	-webkit-flex: 0 0 200px;
	flex: 0 0 200px;  
  padding: 10px;
  text-align: center;  
}

<div class="flex-container">
<div class="flex-item">
			<label>ENTRADA</label>
			<input id="begin" class="label datepickerbegin" type="text" value="<?php echo $avui; ?>" />
</div>

<div class="flex-item">
			<label>SALIDA</label>
			<input id="end" class="label datepickerend" type="text" value="<?php echo $dema;?>" />
</div>

<div class="flex-item">
			<select id="alojamiento">
				<option value="1">Parcela</option>
				<option value="2" selected="selected">Bungalow</option>
			</select>

		<input id="lang" type="hidden"  value="es" />
		<input id="track"type="hidden"  value="formularioweb" />
</div>

<div class="flex-item">
			<input class="btn_submit" type="button" value="RESERVAR" onclick="search()" />
</div>  

<div class="flex-item">
		<a href="http://checkin.unixdata.es/#/login/48"><img src="http://playacambrils.com/wp-content/uploads/2016/12/checkin.png" width="100" height="auto" title="Unix Data CheckIn" alt="Unix Data CheckIn" target="_blank"></a>
</div>
</div>