php preg_replace not doing anything

For some reason my preg_replace call is not working, I have check everything I can think of to no avail. 开发者_运维问答Any suggestions?

foreach ($this->vars as $key=>$var)
{
    preg_replace("/\{$key\}/", $var, $this->tempContentXML);
}

vars is an array containing the $key->value that needs to be replaced in the string, tempContentXML is a string containing XML data.

Piece of the string

...<table:table-cell table:style-name="Table3.B1" office:value-type="string"><text:p text:style-name="P9">{Reference}</text:p></table:table-cell></table:table-row><table:table-row table:style-name="Table3.1"><...

EX.

$this->vars['Reference'] = Test;
foreach ($this->vars as $key=>$var)
{
    preg_replace("/\{$key\}/", $var, $this->tempContentXML);
}

That should replace the string {Reference} with the value in the array at $key

But it is not working.

The replacement does not happen in-place (the new string returned).

foreach ($this->vars as $key=>$var) {
    $this->tempContentXML = preg_replace("/\{$key\}/", $var, $this->tempContentXML);
}

Besides that, don't use a regex for plain string replacements ever (assuming $this->vars does not contain regexes):

foreach ($this->vars as $key=>$var) {
    $this->tempContentXML = str_replace('{'.$key.'}', $var, $this->tempContentXML);
}