How to get everything after a certain character?

Here is the method by using explode:

$text = explode('_', '233718_This_is_a_string', 2)[1]; // Returns This_is_a_string

or:

$text = end((explode('_', '233718_This_is_a_string', 2)));

By specifying 2 for the limit parameter in explode(), it returns array with 2 maximum elements separated by the string delimiter. Returning 2nd element ([1]), will give the rest of string.

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Here is another one-liner by using strpos (as suggested by @flu):

$needle = '233718_This_is_a_string';
$text = substr($needle, (strpos($needle, '_') ?: -1) + 1); // Returns This_is_a_string

The strpos() finds the offset of the underscore, then substr grabs everything from that index plus 1, onwards.

$data = "123_String";    
$whatIWant = substr($data, strpos($data, "_") + 1);    
echo $whatIWant;

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If you also want to check if the underscore character (_) exists in your string before trying to get it, you can use the following:

if (($pos = strpos($data, "_")) !== FALSE) { 
    $whatIWant = substr($data, $pos+1); 
}

strtok is an overlooked function for this sort of thing. It is meant to be quite fast.

$s = '233718_This_is_a_string';
$firstPart = strtok( $s, '_' );
$allTheRest = strtok( '' ); 

Empty string like this will force the rest of the string to be returned.

NB if there was nothing at all after the '_' you would get a FALSE value for $allTheRest which, as stated in the documentation, must be tested with ===, to distinguish from other falsy values.