What is the equivalent of JavaScript’s encodeURIcomponent in PHP?

Try rawurlencode. Or to be more precise:

function encodeURIComponent($str) {
    $revert = array('%21'=>'!', '%2A'=>'*', '%27'=>"'", '%28'=>'(', '%29'=>')');
    return strtr(rawurlencode($str), $revert);
}

This function works exactly how encodeURIComponent is defined:

encodeURIComponent escapes all characters except the following: alphabetic, decimal digits, - _ . ! ~ * ' ( )