get next and previous day with PHP

date('Y-m-d', strtotime('+1 day', strtotime($date)))

Should read

date('Y-m-d', strtotime(' +1 day'))

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Update to answer question asked in comment about continuously changing the date.

<?php
$date = isset($_GET['date']) ? $_GET['date'] : date('Y-m-d');
$prev_date = date('Y-m-d', strtotime($date .' -1 day'));
$next_date = date('Y-m-d', strtotime($date .' +1 day'));
?>

<a href="?date=<?=$prev_date;?>">Previous</a>
<a href="?date=<?=$next_date;?>">Next</a>

This will increase and decrease the date by one from the date you are on at the time.

Simply use this

echo date('Y-m-d',strtotime("yesterday"));
echo date('Y-m-d',strtotime("tomorrow"));

Use

$time = time();

For previous day -

date("Y-m-d", mktime(0,0,0,date("n", $time),date("j",$time)- 1 ,date("Y", $time)));

For 2 days ago

date("Y-m-d", mktime(0,0,0,date("n", $time),date("j",$time) -2 ,date("Y", $time)));

For Next day -

date("Y-m-d", mktime(0,0,0,date("n", $time),date("j",$time)+ 1 ,date("Y", $time)));

For next 2 days

date("Y-m-d", mktime(0,0,0,date("n", $time),date("j",$time) +2 ,date("Y", $time)));

Requirement: PHP 5 >= 5.2.0

You should make use of the DateTime and DateInterval classes in Php, and things will turn to be very easy and readable.

Example: Lets get the previous day.

// always make sure to have set your default timezone
date_default_timezone_set('Europe/Berlin');

// create DateTime instance, holding the current datetime
$datetime = new DateTime();

// create one day interval
$interval = new DateInterval('P1D');

// modify the DateTime instance
$datetime->sub($interval);

// display the result, or print_r($datetime); for more insight 
echo $datetime->format('Y-m-d');


/** 
* TIP:
* if you dont want to change the default timezone, use
* use the DateTimeZone class instead.
*
* $myTimezone = new DateTimeZone('Europe/Berlin');
* $datetime->setTimezone($myTimezone); 
*
* or just include it inside the constructor 
* in this form new DateTime("now",   $myTimezone);
*/

References: Modern PHP, New Features and Good Practices By Josh Lockhart