How to remove characters from a string? - php

Question

(my first post was not clear and confusing so I've edited the question)
I was studying string manipulation.
You can use strlen() or substr() but cannot rely on other functions that are predefined in libraries.
Given string $string = "This is a pen", remove "is" so that
return value is "Th a pen" (including 3 whitespaces).
Remove 'is' means if a string is "Tsih", we don't remove it. Only "is" is removed.
I've tried (shown below) but returned value is not correct. I've run test test and
I'm still capturing the delimiter.
Thanks in advance!
function remove_delimiter_from_string(&$string, $del) {
for($i=0; $i<strlen($string); $i++) {
for($j=0; $j<strlen($del); $j++) {
if($string[$i] == $del[$j]) {
$string[$i] = $string[$i+$j]; //this grabs delimiter :(
}
}
}
echo $string . "\n";
}

Answer 1

Clarifying, the original quiestion is not Implement a str_replace, It's remove 'is' from 'this is a pen' without any functions and no extra white spaces between words. The easiest way would be $string[2] = $string[3] = $string[5] = $string[6] = '' but that would leave an extra white space between Th and a (Th[ ][ ]a).
There you go, no functions at all
$string = 'This is a pen';
$word = 'is';
$i = $z = 0;
while($string[$i] != null) $i++;
while($word[$z] != null) $z++;
for($x = 0; $x < $i; $x++)
for($y = 0; $y < $z; $y++)
if($string[$x] === $word[$y])
$string[$x] = '';

Answer 2

If you were allowed to use substr() it'd be so much easier. Then you could just loop it and check for the matched value, why can't you use substr() but you can strlen() ?
But without, this works at least:
echo remove_delimiter_from_string("This is a pen","is");
function remove_delimiter_from_string($input, $del) {
$result = "";
for($i=0; $i<strlen($input); $i++) {
$temp = "";
if($i < (strlen($input)-strlen($del))) {
for($j=0; $j<strlen($del); $j++) {
$temp .= $input[$i+$j];
}
}
if($temp == $del) {
$i += strlen($del) - 1;
} else {
$result .= $input[$i];
}
}
return $result;
}

Answer 3

The following code can also used to replace the sub string:
$restring = replace_delimiter_from_string("This is a pen","is", "");
var_dump($restring);
$restring = replace_delimiter_from_string($restring," ", " ");
var_dump($restring);
function replace_delimiter_from_string($input, $old, $new) {
$input_len = strlen($input);
$old_len = strlen($old);
$check_len = $input_len-$old_len;
$result = "";
for($i=0; $i<=$check_len;) {
$sub_str = substr($input, $i, $old_len);
if($sub_str === $old) {
$i += $old_len;
$result .= $new;
}
else {
$result .= $input[$i];
if($i==$check_len) {
$result = $result . substr($input, $i+1);
}
$i++;
}
}
return $result;
}