41.php

<?php
function p ($s) {
    echo $s . "\n";
}

$str = 'a:b:c:d:e\:f\\\\:g';
function spl ($str, $delim, $escape='\\') {
    if (strlen ($delim) != 1) {
        return;
    }

    if (strlen ($str) == 0) {
        return array ();
    }

    $ret = array ();
    $idx = 0;
    $idx_prev = 0;
    $start_chunk = 0;
    $is_delim = strlen ($delim);

    while (1) {
        if (($idx = strpos ($str, $delim, $idx_prev)) === false) {
            $ret[] = substr ($str, $start_chunk);
            break;
        }

        $chunk = substr ($str, $start_chunk, $idx - $start_chunk);
        $esc_len = strlen ($chunk) - strlen (rtrim ($chunk, $escape));
        if (!$is_delim || !$esc_len || ($esc_len % 2) == 0) {
            $ret[] = $chunk;
            $start_chunk = $idx + 1;
        }

        $idx_prev = $idx + 1;
    }

    return $ret;
}

$splitted = spl ($str, ':');

p ($str);
print_r($splitted);