Description:
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This might be either string handling related, or date-specific string handling related, but starting with PHP 5.2.1 and still occurring in PHP 5.6.0alpha2, date() (as well as DateTime::format()) does not accept "\0" (NUL) as valid formatting character as valid and truncates at that point.

Likely it means that at some point date() started treating $format value as null-terminated string. Before PHP 5.2.1 the NUL byte was preserved in formatting output.

Reason behind using such characters: to have some clear and easily distinguishable separator which doesn't take up visual space in output.

Test script:
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<?php

date_default_timezone_set( 'UTC' );
$time = strtotime( '2014-01-31' );

$format = "\\Fm\0 j, Y";

var_dump( date( $format, $time ) );

$datetime = new DateTime( '@' . $time );
var_dump( $datetime->format( $format ) );

Expected result:
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string(12) "F01 31, 2014"
string(12) "F01 31, 2014"

Actual result:
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string(3) "F01"
string(3) "F01"