Simple hack based on previous script that allows setting the starting string. It's also quite likely not the ideal and fastest solution

bf2.php

// Set max string length and charset for generating combinations
$maxLength = 3;
$charSet = 'abcdph';
$size = strlen($charSet);

function initBase ($init, $set) {
	$initBase = str_split(strrev($init));
	$positions = array();
	foreach ($initBase as $letter) {
		if ( strpos($set, $letter ) === false) {
			echo "No character '$letter' in given charSet! Please update the charset.<br>";
			return false;
		} else {
			$positions[] = strpos($set, $letter);
		}
	}
	
	return $positions;
}


$counter = 0;
$base = initBase('php', $charSet);
$baseSize = count($base);
$firstRun = true;

while($baseSize <= $maxLength) {
	// Go through all the possible combinations of last character and output $base
	if ($firstRun) $i = $base[0];
	else $i = 0;
	for(; $i < $size; $i++) {
		$base[0] = $i;
		for($j = $baseSize - 1; $j >= 0; $j--) {
			echo $charSet[$base[$j]];
		}
		echo '<br/>';	
	}
	$firstRun = false;
	// How many $base elements reached their max?
	for($i = 0; $i < $baseSize; $i++) {
		if($base[$i] == $size-1) $counter++;
		else break;
	}
	// Every array element reached max value? Expand array and set values to 0. 
	if($counter == $baseSize) {
		// Notice <=$baseSize! Initialize 0 values to all existing array elements and ADD 1 more element with that value
		for($i = 0; $i <= $baseSize; $i++) {
			$base[$i] = 0;
		}
		$baseSize = count($base);
	}
	// Carry one
	else {
		$base[$counter]++;
		for($i = 0; $i < $counter; $i++) $base[$i] = 0;
	}

	$counter=0;
}