<?php
 
    function own_strlen($str) {
        $count = 0;
        while(@$str[$count] != "")
            $count++;
        return $count;
    }
 
    function removeNonNumericalCharacters($str) {
        $result = "";
 
        for($count = 0; $count < own_strlen($str); $count++) {
            $character = $str[$count];
            if((string)(int)$str[$count] === $character)
                $result .= $str[$count];
        }
 
        return $result;
 
    }
 
    $string  = "(123) 011 - 34343678";
    echo removeNonNumericalCharacters($string);
 

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