word_middle_part_replacer.php

<?php

// How to replace middle part of the word

// Array of words
$array = array('what', 'aaa', 'hellokity');

// Function to return replaced value
// Accept value as a string and
// replacer as a character to replace each char in middle part of the word
// for example hellokity => hel000ity
function convert_string($value, $replacer) {
    $len = strlen($value);
    $third = $len / 3;
    $middle_len = ceil($third);
    $prefix_len = floor($third);
    $prefix = substr($value, 0, $prefix_len);
    $suffix = substr($value, -$prefix_len);
    return $prefix . str_repeat($replacer, $middle_len) . $suffix;
}

// Convert each word and output resuts
foreach ($array as $value) {
    echo convert_string($value, '0') . PHP_EOL;
}