performing multiple preg_replace with different search & replace each time

preg_replace can take an array just like str_replace

$string = 'I have a match1 and a match3, and here\'s a match2';
$find = ['/match1/', '/match2/'];
$replace = ['foo', 'bar'];

$result = preg_replace($find, $replace, $string);

You can also use T-Regx library

<?php
$stylesheet = file_get_contents('temp/'.$user.'/css/mobile.css');

$cssTag = 'bodybg';
$stylesheet = pattern("(/\*" . $cssTag . "\*/).*?(/\*/" . $cssTag . "\*/)", 'i')
    ->replace($stylesheet)
    ->all()
    ->by()
    ->map([
        'searchforme' => 'replacewithme',
        'searchforme1' => 'replacewithme2',
        'searchforme1' => 'replacewithme2',
    ]);

You don't need delimiters /, because T-Regx automatically adds them