wp-image-manipulation.php

<?php

$file = '/path/to/image.png';

$editor = wp_get_image_editor( $file );

// true for hard crop
$editor->resize( 300, 300, true );

// use crop for more advanced cropping, positioning etc.
// $editor->crop( $src_x, $src_y, $src_w, $src_h, $dst_w = null, $dst_h = null, $src_abs = false );

// Save file as '/path/to/image-300x300.png'
$new_image_info = $editor->save();