PHP在某些位置将字符串添加到字符串-issue

I'm trying to insert text to string at certain positions (before letter "b" in this case), but for some reason the code I have only insert text (" test ")at the last position/occurance.

<?php
$str = "aabaaaaabaaaaab";
$needle = "b";
$teststr = " test ";
$lastPos = 0;
$positions = array();


while (($lastPos = strpos($str, $needle, $lastPos))!== false) {
$positions[] = $lastPos;
$lastPos = $lastPos + strlen($needle);
}

for ($i=0;$i<count($positions);$i++) {
$newstring = substr_replace($str,$teststr,$positions[$i],0);
}

echo $newstring;
?>`

this produces the following output: aabaaaaabaaaaa test b when the desired one whould be: aa test aaaaa test aaaaa test b

You use $str as input for substring_replace, but you don't modify $str anywhere. Obviously only the last replacement will show. You can, for example, use $newstring as input for substring_replace, but then you positions are no longer correct. This can be avoided by making the replacements from right to left:

//snip

$newstring = $str;
for ($i = count($positions) - 1; $i >= 0; $i--) {
  $newstring = substr_replace($newstring, $teststr, $positions[$i], 0);
}

echo $newstring;

Would regular expressions work for you?

<?php
$str = "aabaaaaabaaaaab";
echo preg_replace('~b~', ' test b', $str);

$str = "aabaaaaabaaaaab";
$needle = "b";
$teststr = " test ";
$lastPos = 0;
$positions = explode($needle, $str);
foreach($positions as $k=>$v) {
    $positions[$k]=$v.$teststr.$needle;
}
$positions=implode($positions);
echo $positions;

Try This

Following should work

$str = "aabaaaaabaaaaab";
$needle = "b";
$teststr = " test ";

for ($i=0;$i<strlen($str);$i++) {
    if($str[$i]==$needle ){
        echo $teststr.$str[$i]; 
    }else{
        echo $str[$i];
    }
}

echo $newstring;

**Output** 
aa test baaaaa test baaaaa test b