PHP Autoboxing: What it could look like I don't think, multiple autoboxing-functions make sense, a __autobox would indeed be more useful than spl_autobox_register, since you can't "merge" multiple classes defined on autoboxes and frameworks require specific functions. The best way would be enabling/disabling the autoboxing in frameworks, set it …

Autoboxing.php

<?php

class Type {
    
    protected $value;
    
    public function __construct( $value ) { $this->value = $value; }
    abstract public function __cast( /* (string) Class name */ $type );
}

class String extends Type {
    
    public function toUppercase() {
        
        $this->value = strtoupper( $this->value );
        
        return $this;
    }
    
    public function toLowercase() {
        
        $this->value = strtolower( $this->value );
        
        return $this;
    }
    
    public function toArray() {
        
        for( $i = 0; $i < strlen( $this->value ); $i++ )
            yield $this->value[ $i ];
    }
    
    public function __cast( $type ) {
        
        switch( $type ) {
            case 'Integer':
                
                return intval( $this->value );
            case 'Array':
                
                return $this->toArray();
        }
    }
}

class Integer extends Type {
    
    public function add( $int ) { $this->value += $int; return $this; }
    public function sub( $int ) { $this->value -= $int; return $this; }
    public function div( $int ) { $this->value /= $int; return $this; }
    public function mul( $int ) { $this->value *= $int; return $this; }
    
    public function rangeFrom( $int ) {
        
        for( $i = $int; $i < $this->value; $i++ )
            yield $i;
    }
    
    public function rangeTo( $int ) {
        
        for( $i = $this->value; $i < $int; $i++ )
            yield $i;
    }
    
    public function __cast( $type ) {
        
        switch( $type ) {
            case 'String':
                
                return strval( $this->value );
            case 'Array':
                
                return $this->rangeFrom( 0 );
        }
    }
}

class Array {
    
    public function forEach( $callback ) {
        
        foreach( $this->value as $k => $val ) {
            $result = call_user_func( $callback, $val, $k );
            
            if( !is_null( $result ) )
                $this->value[ $k ] = $result;
        }
        
        return $this;
    }
    
    public function __cast( $type ) {
        
        switch( $type ) {
            case 'String':
                
                $str = '';
                $this->forEach( function( $v ) use( $str ) { $str .= $v } );
                return $str;
            case 'Integer':
                
                throw new Exception( "Invalid cast from array to integer (how shud i do eet?)" );
                
        }
    }
}



/* Autoboxing initialization */

if( function_exists( '__autobox' ) )
    throw new Exception( "Sorry, autoboxing can only be used ONCE per project. "
        . "Anything else would make no sense, other than with __autoload" );

function __autobox( $primitiveType, $value ) {
    
    switch( $primitiveType ) {
        case 'string':
            
            return new String( $value );
        case 'int':
            
            return new Integer( $value );
        case 'array':
            
            return new Array( $value );
    }
}




/* Possibilities */

$str = 'my string';
var_dump( $str->toUppercase() ); //str.value = 'MY STRING'
var_dump( $str->toLowercase()->toUppercase() ); //str.value = 'MY STRING'

$int = 14;
var_dump( $int->mul( 2 ) ); //int.value = 28
var_dump( $int->rangeFrom( 5 ) ); //(Array)[5,6,7,...,27,28]

$arr = [ 0, 1, 2, 3, 4, 5 ];

$arr->forEach( function( $i ) {
    
    echo $i;
    
    //!! return value could re-map the values easily
    return $i * $i;
} );
var_dump( $arr ); //arr.value = [0,2,4,9,16,25]



/* Type casting */

$str = 'test';
$arr = (Array)$str;
var_dump( $arr ); //arr.value = ['t','e','s','t']

$int = 5;
$str = (String)$int;
var_dump( $int ); //int.value = '5'

$arr = (Array)$int;
var_dump( $arr ); //arr.value = [0,1,2,3,4,5]

NSAutoboxing.php

<?php
/* This is what __autobox with namespaces could look like */

namespace MyNs;

//this would count for \MyNs and \MyNs\*, all subnamespaces
function __autobox( $primitiveType, $value ) {
  //... 
}

namespace MyNs\SomeSubNs;

//this would overwrite __autobox of \MyNs, but only for \MyNs\SomeSubNs and \MyNs\SomeSubNs\*
function __autobox( $primitiveType, $value ) {
  //...
}