[php] How to check if a string starts with a specified string?

I'm trying to check if a string starts with http. How can I do this check?

$string1 = 'google.com';
$string2 = 'http://www.google.com';

You can use a simple regex (updated version from user viriathus as eregi is deprecated)

if (preg_match('#^http#', $url) === 1) {
    // Starts with http (case sensitive).
}

or if you want a case insensitive search

if (preg_match('#^http#i', $url) === 1) {
    // Starts with http (case insensitive).
}

Regexes allow to perform more complex tasks

if (preg_match('#^https?://#i', $url) === 1) {
    // Starts with http:// or https:// (case insensitive).
}

Performance wise, you don't need to create a new string (unlike with substr) nor parse the whole string if it doesn't start with what you want. You will have a performance penalty though the 1st time you use the regex (you need to create/compile it).

This extension maintains a global per-thread cache of compiled regular expressions (up to 4096). http://www.php.net/manual/en/intro.pcre.php

You can check if your string starts with http or https using the small function below.

function has_prefix($string, $prefix) {
   return substr($string, 0, strlen($prefix)) == $prefix;
}

$url   = 'http://www.google.com';
echo 'the url ' . (has_prefix($url, 'http://')  ? 'does' : 'does not') . ' start with http://';
echo 'the url ' . (has_prefix($url, 'https://') ? 'does' : 'does not') . ' start with https://';

Use strpos():

if (strpos($string2, 'http') === 0) {
   // It starts with 'http'
}

Remember the three equals signs (===). It will not work properly if you only use two. This is because strpos() will return false if the needle cannot be found in the haystack.

There is also the strncmp() function and strncasecmp() function which is perfect for this situation:

if (strncmp($string_n, "http", 4) === 0)

In general:

if (strncmp($string_n, $prefix, strlen($prefix)) === 0)

The advantage over the substr() approach is that strncmp() just does what needs to be done, without creating a temporary string.