Parsing text and return hostname before period with PHP

$hostname = "abc.domain.com"

I just want "abc" and nothing after开发者_运维知识库 it.

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With substr and strpos:

$host = substr($hostname, 0, strpos($hostname, '.'));

or maybe better, strstr:

$host = strstr($hostname, '.', true);

There are a lot of functions available to process strings.

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Use explode():

$parts = explode('.', $hostname);
// $parts[0]

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Will it always have a subdomain?

If so, you can just do

$parts = explode('.', $hostname);
$subdomain = $parts[0];

If there might not be a subdomain

$parts = explode('.', $hostname);
$subdomain = count($parts) == 3 ? $parts[0] : NULL;