Here is a simple one liner to repeat a string multiple times with a separator:

<?php
implode($separator, array_fill(0, $multiplier, $input));
?>

Example script:
<?php

// How I like to repeat a string using standard PHP functions
$input = 'bar';
$multiplier = 5;
$separator = ',';
print implode($separator, array_fill(0, $multiplier, $input));
print "\n";

// Say, this comes in handy with count() on an array that we want to use in an
// SQL query such as 'WHERE foo IN (...)'
$args = array('1', '2', '3');
print implode(',', array_fill(0, count($args), '?'));
print "\n";
?>

Example Output:
bar,bar,bar,bar,bar
?,?,?

http://php.net/manual/en/function.str-repeat.php#90555

Damien Bezborodov , yeah but execution time of your solution is 3-5 times worse than str_replace.

<?php

function spam($number) {
    return str_repeat('test', $number);
}

function spam2($number) {
    return implode('', array_fill(0, $number, 'test'));
}

//echo spam(4);
$before = microtime(true);
for ($i = 0; $i < 100000; $i++) {
    spam(10);
}
echo microtime(true) - $before , "\n"; // 0.010297
$before = microtime(true);
for ($i = 0; $i < 100000; $i++) {
    spam2(10);
}
echo microtime(true) - $before; // 0.032104

Here is a shorter version of Kees van Dieren's function below, which is moreover compatible with the syntax of str_repeat:

<?php
function str_repeat_extended($input, $multiplier, $separator='')
{
    return $multiplier==0 ? '' : str_repeat($input.$separator, $multiplier-1).$input;
}
?>

hi guys , 
i've faced this example :
<?php

$my_head = str_repeat("°~", 35);
echo $my_head;

?>

so , the length should be 35x2 = 70 !!!
if we echo it :

<?php
$my_head = str_repeat("°~", 35);
echo strlen($my_head); // 105
echo mb_strlen($my_head, 'UTF-8'); // 70
?>

be carefull with characters and try to use mb_* package to make sure everything goes well ...