Description:
------------
Date functions/methods like date_parse_from_format(), date_create_from_format()/DateTime::createFromFormat() ignore leading zeros in format and date string. So doe not make sense you use, for example, 'm' or 'n', both '01' and '1' passed without errors or warnings.
Looks relevant to all versions from 5.2 to 5.6.

Test script:
---------------
<?php

// date with leading zeros
$d = '1990-01-02';
// format without leading zeros
$f = 'Y-n-j';
print_r(date_parse_from_format($f, $d));
print_r(date_create_from_format($f, $d));
print_r(DateTime::getLastErrors());
echo '--------------------------';
// date without leading zeros
$d = '1990-1-2';
// format with leading zeros
$f = 'Y-m-d';
print_r(date_parse_from_format($f, $d));
print_r(date_create_from_format($f, $d));
print_r(DateTime::getLastErrors());

?>

Expected result:
----------------
Expect some warnings at least.

Actual result:
--------------
Array
(
    [year] => 1990
    [month] => 1
    [day] => 2
    [hour] => 
    [minute] => 
    [second] => 
    [fraction] => 
    [warning_count] => 0
    [warnings] => Array
        (
        )

    [error_count] => 0
    [errors] => Array
        (
        )

    [is_localtime] => 
)
DateTime Object
(
    [date] => 1990-01-02 05:00:38.000000
    [timezone_type] => 3
    [timezone] => America/New_York
)
Array
(
    [warning_count] => 0
    [warnings] => Array
        (
        )

    [error_count] => 0
    [errors] => Array
        (
        )

)
--------------------------
Array
(
    [year] => 1990
    [month] => 1
    [day] => 2
    [hour] => 
    [minute] => 
    [second] => 
    [fraction] => 
    [warning_count] => 0
    [warnings] => Array
        (
        )

    [error_count] => 0
    [errors] => Array
        (
        )

    [is_localtime] => 
)
DateTime Object
(
    [date] => 1990-01-02 05:00:38.000000
    [timezone_type] => 3
    [timezone] => America/New_York
)
Array
(
    [warning_count] => 0
    [warnings] => Array
        (
        )

    [error_count] => 0
    [errors] => Array
        (
        )

)