How to compare dates in PHP?

To be able to work with date first of all we have to keep in mind that the standard used is the American Standard which requires us to use the Year-Month-Day format (ex.: 2013-05-22).

In our example we will create a script that compares whether data1 and greater than or equal to data2 and displaying the corresponding messages.

<?php
 $data1 = '2013-05-21';
 $data2 = '2013-05-22';

 // Comparando as Datas
 if(strtotime($data1) > strtotime($data2))
 {
  echo 'A data 1 é maior que a data 2.';
 }
 elseif(strtotime($data1) == strtotime($data2))
 {
  echo 'A data 1 é igual a data 2.';
 }
 else
 {
  echo 'A data 1 é menor a data 2.';
 }
?>

Obs.: The strtotime command generates the timestamp of a date in textual format so that we can work with the dates.

Already for the conversion of pattern can be done this way:

$dataString = '19/03/2013 11:22';
$date = DateTime::createFromFormat('d/m/Y H:i', $dataString);
echo $date->format('Y-m-d H:i:s');

And of course it must be adapted to your need.

A quick and easy way to do this is by using the Class DateTime together with the method createFromFormat.

$timeZone = new DateTimeZone('UTC');

/** Assumido que $dataEntrada e $dataSaida estao em formato dia/mes/ano */
$data1 = DateTime::createFromFormat ('d/m/Y', $dataEntrada, $timeZone);
$data2 = DateTime::createFromFormat ('d/m/Y', $dataSaida, $timeZone);

/** Testa se sao validas */
if (!($data1 instanceof DateTime)) {
  echo 'Data de entrada invalida!!';
}

if (!($data2 instanceof DateTime)) {
  echo 'Data de saida invalida!!';
}

/** Compara as datas normalmente com operadores de comparacao < > = e !=*/
if ($data1 > $data2) {
  echo 'Data de entrada maior que data de saida!';
}

if ($data1 < $data2) {
  echo 'Data de entrada menor que data de saida!';
}

In this example I used the Class DateTimeZone to ensure that the dates are in the same time zone, avoiding schedule problems.

In addition to what Otto has already said, You can do this in a less specific way (which is what I thought you wanted, since you wrote that it did not fit the American Standard). If you save the date with dashes or delimiters in the middle of it, just use preg_replace (or any other function that fulfills this) in the string to remove the character that separates year, month and day. If you do not use go straight to the next step...

After removing the bounding characters:

<?php
$data_entrada = "01022014";
$dia_entrada = substr($data_entrada, 0, 2);
$mes_entrada = substr($data_entrada, 2, 2);
$ano_entrada = substr($data_entrada, 4, 4);
$data_saida = "01022014";
$dia_saida = substr($data_saida, 0, 2);
$mes_saida = substr($data_saida, 2, 2);
$ano_saida = substr($data_saida, 4, 4);


if ($ano_saida > $ano_entrada) {
    echo "A data de saída é posterior a de entrada";
} elseif ($ano_saida == $ano_entrada) {
// CASO ANO IGUAL
    if ($mes_saida > $mes_entrada) {
        echo "A data de saída é posterior a de entrada";
    } elseif ($mes_saida == $mes_entrada) {
        // INICIO CASO MES IGUAL
       if ($dia_saida > $dia_entrada) {
        echo "A data de saída é posterior a de entrada";
       } elseif ($dia_saida == $dia_entrada) {
        echo "As datas de saída e entrada são  iguais";
} elseif ($dia_saida < $dia_entrada) {
    echo "A data de saída é anterior a de entrada";
} // FIM CASO MES IGUAL
} elseif ($mes_saida < $mes_entrada) {
    echo "A data de saída é anterior a de entrada";
}
// FIM DO CASO ANO IGUAL
} else {
    echo "A data de saída é anterior a de entrada";
}

?>

Note: If if necessary, remember to remove whitespace after and before the string using trim() or others...