This function will give you the number of days between March 21 and Easter of the year given. Below is a quote from the manual explaining why it makes any sense to do so.

Unlike easter_date(), this function is not restricted by the Unix timestamp so now you can calculate the Easters prior to 1970 all you want. But like easter_date(), the year is optional here too.

The date of Easter Day was defined by the Council of Nicaea in AD325 as the Sunday after the first full moon which falls on or after the Spring Equinox. The Equinox is assumed to always fall on 21st March, so the calculation reduces to determining the date of the full moon and the date of the following Sunday.

$easter = easter_days();     // 22
$easter = easter_days(1985); // 17
$easter = easter_days(2032); // 7
 
$easter = easter_days(1863); // 15
$easter = easter_days(2517); // 21

There’s a second optional parameter to set which calendar to use. This is an issue because the Gregorian calendar was in use during the years 1582 - 1752. For that case you can pass in the constant CAL_EASTER_ROMAN

$easter = easter_days(1625);                   // 27
$easter = easter_days(1625, CAL_EASTER_ROMAN); // 9